# Frank Solutions for Chapter 3 Compound Interest Class 9 Mathematics ICSE

1. Find the amount and the compound interest payable annually on the following:

(i) Rs 25000 for 1.1/2 years at 10% per annum.

(ii) Rs 32000 for 2 years at 7.1/2 % per annum.

(iii) Rs 10000 for 2.1/2 years at 6% per annum.

(iv) Rs 24000 for 1.1/2 years at 7.1/2% per annum.

(i) Rs 25000 for 1.1/2 years at 10% per annum.

Here,

P = Rs 25000, t = 1.1/2 years, r = 10%

Now,

Amount after 1 year = P (1 + r/100)

= 25000 (1 + 10/100)

= 25000 (1 + 1/10)

On further calculation, we get,

= 25000 (11/10)

= 27500

Hence, principle for the next 6 months = Rs 27500

Interest for the next 6 months = (27500 × 6 × 10)/(100 × 12)

= 1375

Hence, amount after 1.1/2 years = Rs 27500 + Rs 1375

= Rs 28875

And CI = A – P

= Rs 28875 – Rs 25000

= Rs 3875

(ii) Rs 32000 for 2 years at 7.1/2% per annum.

Here,

P1 = Rs 32000 and r = 7.1/2 % = (15/2)%

So, Amount after 1 year = P (1 + r/100)

= 32000 {1 + 15/(2 × 100)}

= 32000 (1 + 3/40)

= 32000 (43/40)

We get,

= 34400

Therefore,

P2 = Rs 34400 and r = (15/2)%

So, Amount after 2 year = P (1 + r/100)

= 34400 {1 + 15/(2 × 100)}

On further calculation, we get,

= 34400 (1 + 3/40)

= 34400 (43/40)

We get,

= 36980

Hence, Amount = Rs 36980

And CI = A – P

= Rs 36980 – Rs 32000

We get,

= Rs 4980

(iii) Rs 10000 for 2.1/2 years at 6% per annum.

Here,

P1 = Rs 10000 and r = 6%

So, Amount after 1 year = P (1 + r / 100)

= 10000 (1 + 6/100)

= 10000 (106/100)

On simplification, we get,

= 10600

Hence, P2 = Rs 10600 and r = 6%

Amount after 2 year = P (1 + r/100)

= 10600 (1 + 6/100)

= 10600 (106/100)

On simplification, we get,

= Rs 11236

Hence, principle for the next 6 months = Rs 11236

Interest for the next 6 months = (11236 × 6 × 6)/(100 × 12)

= 337.08

Hence, amount after 1.1/2 years = Rs 11236 + Rs 337.08

= Rs 11573.08

And CI = A – P

= Rs 11573.08 – Rs 10000

= Rs 1573.08

(iv) Rs 24000 for 1.1/2 years at 7.1/2 % per annum.

Here,

P = Rs 24000, t = 1.1/2 years, r = 7.1/2 = (15/2)%

Now,

Amount after 1 year = P (1 + r/100)

= 24000 {1 + 15/(2 × 100)}

= 24000 (1 + 3/40)

= 24000 (43/40)

We get,

= 25800

Hence, principle for the next 6 months = Rs 25800

Interest for the next 6 months = (25800 × 15 × 6)/(200 × 12)

= 967.50

Hence, amount after 1.1/2 years = Rs 25800 + Rs 967.50

= 26767.50

And CI = A – P

= Rs 26767.50 – Rs 24000

= Rs 2767.50

2. Find the amount and the compound interest payable annually on:

(a) Rs 16000 for 2 years at 15% and 12% for the successive years.

(b) Rs 17500 for 3 years at 8%, 10% and 12% for the successive years

(a) For first year: P = Rs 16000, R = 15% and T = 1 year

Therefore, interest = Rs (16000× 15 × 1)/100

= Rs 2400

And, amount = Rs 16000 + Rs 2400

= Rs 18400

For second year: P = Rs 18400, R = 12% and T = 1 year

Therefore, interest = Rs (18400 × 12 × 1)/100

= Rs 2208

And, amount = Rs 18400 + Rs 2208

= Rs 20608

Hence, required amount = Rs 20608

And, Compound Interest = A – P

= Rs 20608 – Rs 16000

We get,

= Rs 4608

(b) For first year: P = Rs 17500, R = 8% and T = 1 year

Therefore, interest = Rs (17500 × 8 × 1)/100

= Rs 1400

And, amount = Rs 17500 + Rs 1400

= Rs 18900

For second year: P = Rs 18900, R = 10% and T = 1 year

Therefore, interest = Rs (18900 × 10 × 1)/100

= Rs 1890

And, amount = Rs 18900 + Rs 1890

= Rs 20790

For third year: P = Rs 20790, R = 12% and T = 1 year

Therefore, interest = Rs (20790 × 12 × 1)/100

= Rs 2494.80

And, amount = Rs 20790 + Rs 2494.80

= Rs 23284.80

Hence, required amount = Rs 23284.80

And, Compound Interest = A – P

= Rs 23284.80 – Rs 17500

= Rs 5784.80

3. Calculate the amount and compound interest on Rs 20000 for 3 years at 10% per annum, interest being payable annually.

Here,

P= Rs 20000 and r = 10%

So, amount after 1 year = P (1 + r/100)

= 20000 (1 + 10/100)

= 20000 (110/100)

We get,

= 22000

Hence, P2 = Rs 22000 and r = 10%

Amount after 2 year = P (1 + r/100)

= 22000 (1 + 10/100)

= 22000 (110/100)

We get,

= 24200

Hence, P3 = Rs 24200 and r = 10%

Amount after 3 year = P (1 + r/100)

= 24200 (1 + 10/100)

= 24200 (110/100)

We get,

= 26620

Therefore, amount = Rs 26620

Also, CI = A – P

= Rs 26620 – Rs 20000

= Rs 6620

4. Compute the compound interest for the third year on Rs 5000 invested for 5 years at 10% per annum, the interest being payable annually.

For first year: P = Rs 5000, R = 10% and T = 1 year

Therefore, interest = Rs (5000 × 10 × 1)/100

= Rs 500

And, amount = Rs 5000 + Rs 500

= Rs 5500

For second year: P = Rs 5500, R = 10% and T = 1 year

Therefore, interest = Rs (5500 × 10 × 1)/100

= Rs 550

And, amount = Rs 5500 + Rs 550

= Rs 6050

For third year: P = Rs 6050, R = 10% and T = 1 year

Therefore, interest = Rs (6050 × 10 × 1)/100

= Rs 605

Hence, Compound Interest for third year is Rs 605

5. Rakesh invests Rs 25600 at 5% per annum compound interest payable annually for 3 years. Find the amount standing to his credit at the end of the second year.

For first year: P = Rs 25600, R = 5% and T = 1 year

Therefore, interest = Rs (25600 × 5 × 1)/100

= Rs 1280

And, amount = Rs 25600 + Rs 1280

= Rs 26880

For second year: P = Rs 26880, R = 5% and T = 1 year

Therefore, interest = Rs (26880 × 5 × 1)/100

= Rs 1344

And, amount = Rs 26880 + Rs 1344

= Rs 28224

Therefore, amount at the end of second year is Rs 28224

6. Find the amount and compound interest on Rs 7500 for 1.1/2 years at 8%, payable semi-annually.

Here,

P1 = Rs 7500 and rate of interest for half year (r) = 4%

So, amount after half year = P (1 + r/100)

= 7500 (1 + 4/100)

= 7500 (104/100)

We get,

= 7800

Hence, P2 = Rs 7800 and r = 4%

Amount after 1 year = P (1 + r/100)

= 7800 (1 + 4/100)

= 7800 (104/100)

We get,

= 8112

Hence, P3 = Rs 8112 and r = 4%

Amount after 1.1/2 year = P (1 + r/100)

= 8112 (1 + 4/100)

= 8112 (104/100)

We get,

= 8436.48

Therefore, amount = Rs 8436.48

Also, CI = A – P

= Rs 8436.48 – Rs 7500

= Rs 936.48

7. A man invests Rs 24000 for two years at compound interest, if his money amounts to Rs 27600 after one year, find the amount at the end of second year.

Amount after 1 year = P (1 + r/100)

27600 = 24000 (1 + r/100)

(1 + r/100) = 27600/24000

We get,

(1 + r/100) = 23/20

On further calculation, we get,

r/100 = (23/20) – 1

We get,

r/100 = 3/20

r = (100 × 3)/20

r = 15

Amount after 2 year = P (1 + r/100)

= 27600 {1 + (15/100)}

= 27600 (115/100)

We get,

= 31740

Therefore, the amount at the end of second year is Rs 31740

8. How much will Rs 14000 amounts to 2 years at compound interest, if the rates for the successive years be 5% and 8% respectively?

Here,

P1 = Rs 14000 and r = 5%

So, Amount after 1 year = P (1 + r/100)

= 14000 (1 + 5/100)

= 14000 (105/100)

We get,

= 14700

Hence, P2 = Rs 14700 and r = 8%

Amount after 2 year = P (1 + r/100)

= 14700 (1 + 8/100)

= 14700 (108/100)

We get,

= 15876

Therefore, amount = Rs 15876

9. Find the amount and the compound interest on the following:

(i) Rs 8000 for 3 years at 10% per annum compounded annually

(ii) Rs 15000 for 2 years at 8% per annum compounded semi-annually

(iii) Rs 12000 for 1.1/2 years at 5% per annum compounded annually

(iv) Rs 25000 for 2 years at 6% per annum compounded semi-annually

(v) Rs 16000 for 3 years at 10%, 8% and 6% for successive years

(i) Rs 8000 for 3 years at 10% per annum compounded annually

Here,

P = Rs 8000, t = 3 years, r = 10%

Now,

Amount = P (1 + r/100)t

= 8000 (1 + 10/100)3

= 8000 (11/10)3

= 8000 × (1331/1000)

We get,

= 10648

Therefore, amount = Rs 10648

Also, CI = A – P

= Rs 10648 – Rs 8000

= Rs 2648

(ii) Rs 15000 for 2 years at 8% per annum compounded semi-annually

Here,

P = Rs 15000, t = 2 years, r = 8%

Since interest is compounded semi-annually, so

Amount = P (1 + r/200)2t

= 15000 (1 + 8/200)4

= 15000 (26/25)4

= 15000 × (26/25) × (26/25) × (26/25) × (26/25)

On simplification, we get,

= 17547.88

Therefore, amount = Rs 17547.88

Also, CI = A – P

= Rs 17547.88 – Rs 15000

= Rs 2547.88

(iii) Rs 12000 for 1.1/2 years at 5% per annum compounded annually

Here,

P = Rs 12000, t = 1.1/2 years, r = 5%

Now,

Amount after 1 year = P (1 + r/100)t

= 12000 (1 + 5/100)

On simplification, we get,

= 12000 (105/100)

= 12600

Now, interest for the next half year = (12600 × 5)/(100 × 2)

= 315

Therefore, amount = Rs 12600 + Rs 315

= Rs 12915

Also, CI = A – P

= Rs 12915 – Rs 12000

= Rs 915

(iv) Rs 25000 for 2 years at 6% per annum compounded semi-annually

Here,

P = Rs 25000, t = 2 years, r = 6%

Since interest is compounded semi-annually,

Amount = P (1 + r/200)2t

= 25000 (1 + 6/200)4

= 25000 (103/100)4

On simplification, we get,

= 28137.72

Hence, amount = Rs 28137.72

Also, CI = A – P

= Rs 28137.72 – Rs 25000

= Rs 3137.72

(v) Rs 16000 for 3 years at 10%, 8% and 6% for successive years

Here,

P = Rs 16000, t = 3 years, r = 10%, 8%, 6% successively

Now,

Amount = P (1 + r1/100) (1 + r2/100) (1 + r3/100)

= 16000 (1 + 10/100) (1 + 8/100) (1 + 6/100)

On simplification, we get,

= 16000 (11/10) (108/100) (106/100)

= 20148.48

Therefore, Amount = Rs 20148.48

Also, CI = A – P

= Rs 20148.48 – Rs 16000

= Rs 4148.48

10. Find the amount and compound interest on Rs 15000 in 2.1/2 years at 10% p.a. compounded annually

Here,

P = Rs 15000, t = 2.1/2 years, r = 10%

Now, Amount after 2 year = P (1 + r / 100)t

= 15000 (1 + 10/100)2

= 15000 (11/10)2

We get,

= 18150

Now, interest for the next half year = (18150 × 10)/(100 × 2)

= 907.5

Therefore, Amount = Rs 18150 + Rs 907.50

= Rs 19057.50

Also, CI = A – P

= Rs 19057.50 – Rs 15000

= Rs 4057.50

11. Find the amount on Rs 36000 in 2 years 15% p.a. compounded annually.

Here,

P = Rs 36000, t = 2 years, r = 15%

Now,

Amount = P (1 + r/100)t

= 36000 (1 + 15/100)2

= 36000 (115/100)2

We get,

= 47610

Therefore, amount = Rs 47610

12. Find the amount and compound interest on Rs 50000 in 1.1/2 years at 8% p.a. compounded half-yearly.

Here,

P = Rs 50000, t = 1.1/2 years, r = 8%

Since interest is compounded half-yearly,

So, amount = P (1 + r/200)2t

= 50000 (1 + 8/200)3

= 50000 (104/100)3

We get,

= 56243.20

Therefore, Amount = Rs 56243.20

Also, CI = A – P

= Rs 56243.20 – Rs 50000

= Rs 6243.20

13. How much will Rs 25000 amount to in 2 years at compound interest, if the rates for 1st and 2nd years be 4% and 5% p.a. respectively?

Here,

P = Rs 25000, t = 2 years, r = 4%, 5% successively

Now,

Amount = P (1 + r1/100) (1 + r2/100)

= 25000 (1 + 4/100) (1 + 5/100)

On further calculation, we get,

= 25000 (104/100) (105/100)

= 27300

Therefore, Amount = Rs 27300

14. Find compound interest on Rs 31250 for 3 years, if the rates of interest for 1st, 2ndand 3rdyears be 8%, 10% and 12% respectively.

Here,

P = Rs 31250, t = 3 years, r = 8%, 10%, 12% successively

Now,

Amount = P (1 + r1/100) (1 + r2/100) (1 + r3/100)

= 31250 (1 + 8/100) (1 + 10/100) (1 + 12/100)

On further calculation, we get,

= 31250 (108/100) (110/100) (112/100)

= 41580

Therefore, Amount = Rs 41580

15. Calculate the rate percent when Rs 28000 amount to Rs 30870 in 2 years at compounded annually.

Here,

P = Rs 28000, A = 30870, t = 2 years

Now,

Amount = P (1 + r/100)t

30870 = 28000 (1 + r/100)2

⇒ (1 + r/100)2 = (30870/28000)

We get,

(1 + r/100)2 = (441/400)

⇒ (1 + r/100)2 = (21/20)2

Hence,

(1 + r/100) = (21/20)

⇒ r/100 = (21/20) – 1

⇒ r /100 = 1/20

⇒ r = 100/20

⇒ r = 5

Therefore, rate of interest is 5%

16. Simple interest on a sum of money for 2 years at 4% is Rs 450. Find the compound interest at the same rate for 1 year if the interest is reckoned half-yearly.

17. A man borrows Rs 62500 at 8% p.a., simple interest for 2 years. He immediately lends the money out at CI at the same rate and for same time. What is his gain at the end of 2 years?

18. What sum will amount to Rs 10120 in 2 years at CI payable annually, if the rates are 10% and 15% for the successive years ?

19. Sunil borrows Rs 50,000 at 10% SI for 1.1/2 years. He immediately invests the entire amount for 1.1/2 years at 10% compounded annually. What is his gain at the end of the stipulated time, when he repays his loan?

20. The value of a mobile depreciated by 5% per year during the first two years and 10% per year during the third year. Express the total depreciation of the value of the mobile in percent during the three years.

21. A man borrows Rs 6500 at 10% per annum compound interest payable half-yearly. He repays Rs 2000 at the end of every six months. Calculate the amount outstanding at the end of the third payment. Give your answer to the nearest rupee.

22. A man borrows Rs 20000 at 10% per annum compound interest payable annually. If he repays Rs 5000 at the end of the first year and Rs 10000 at the end of the second year, how much should he pay at the end of the third year in order to clear the account ? Find the answer correct to the nearest rupee.

23. Ankita bought a gold ring worth Rs x. The value of the ring increased at 10% per year compounded annually, on which the appreciation for the first year plus the appreciation for the second year amounts to Rs 6300. Find the value of the ring.

24. Priyanka lends Rs 15,500 at 10% for the first year, at 15% for the second year and at 20% for the third year. If the rates of interest are compounded yearly, find the difference between the compound interest of the second year and the third year.

25. Samidha borrowed Rs 7500 from Shreya at 30% per annum compounded interest. After 2 years, Samidha gave Rs 10000 and a juicer to Shreya to clear the debt. Find the cost of the juicer.

### Exercise 3.2

1. Find the amount and the compound interest on the following:

(i) Rs 8000 for 3 years at 10% per annum compounded annually.

(ii) Rs 15000 for 2 years at 8% per annum compounded semi-annually.

(iii) Rs 12000 for 1.1/2 years at 5% per annum compounded annually.

(iv) Rs 25000 for 2 years at 6% per annum compounded semi-annually.

(v) Rs 16000 for 3 years at 10%, 8% and 6% for successive years.

(i)

(ii)
(iii)
(iv)
(v)

2. Find the amount and compound interest on Rs 15000 in 2.1/2 years at 10% p.a. compounded annually.

3. Find the amount on Rs 36000 in 2 years 15% p.a. compounded annually.

4. Find the amount and compound interest on Rs 50000 in 1.1/2 years at 8% p.a. compounded half-yearly.

5. How much will Rs 25000 amount to in 2 years at compound interest, if the rates for 1st and 2nd years be 4% and 5% p.a. respectively ?

6. Find compound interest on Rs 31250 for 3 years, if the rates for 1st, 2nd and 3rd years be 8%, 10% and 12% respectively.

7. Calculate the rate when Rs 28000 amount to Rs 30870 in 2 years at compounded annually.

8. In what time Rs 15625 amount to Rs 17576 at 4% p.a. compound interest ?

9. In how many years will Rs 2000 amount to Rs 2662 at 10% p.a. compound interest ?

10. The simple interest on a certain sum for 3 years at 4% is RS 600. Find the compound interest for the same sum at same percent and in the same time.

11. The compound interest payable annually on a certain sum for 2 years in Rs 40.80 and the simple interest is Rs 40. Find the sum and the rate percent.

12. The difference between simple interest and compound interest compounded annually on a certain sum is Rs 448 for 2 years at 8 percent per annum. Find the sum.

13. The difference between CI payable annually and SI on Rs 50,000 for two years is Rs 125 at the same rate of interest per annum. Find the rate of interest.

14. What principal will amount to Rs 15729 in two years, if the rate of interest for successive years are 5% and 7% respectively, the interest is being compounded annually.