## Compound Interest Questions and Answers Part-1

1. A sum becomes Rs.1352 in 2 years at 4% per annum compound interest. The sum is =
a) Rs. 1270
b) Rs. 1225
c) Rs. 1245
d) Rs. 1250

Explanation: Let the sum be Rs. x
\eqalign{ & \therefore 1352 = x{\left( {1 + \frac{4}{{100}}} \right)^2} \cr & \Rightarrow 1352 = x{\left( {1 + \frac{1}{{25}}} \right)^2} \cr & \Rightarrow 1352 = x{\left( {\frac{{26}}{{25}}} \right)^2} \cr & \Rightarrow x = \frac{{1352 \times 25 \times 25}}{{26 \times 26}} \cr & \Rightarrow x = {\text{Rs}}{\text{.}}\,1250 \cr}

2. What will be the compound interest accrued on an amount of Rs.10000 @ 20 p.c.p.a in 2 years if the interest is compounded half-yearly?
a) Rs. 4600
b) Rs. 4641
c) Rs. 4400
d) Rs. 4680

Explanation:
\eqalign{ & {\text{P = Rs}}.10000, \cr & {\text{R}} = 20\% \,p.a. \cr & \,\,\,\,\,\,\, = 10\% \,{\text{per}}\,{\text{half year}} \cr & T = 2\,{\text{years}} = 4\,{\text{half}}\,{\text{years}} \cr & {\text{Amount}} \cr & {\text{ = Rs}}.\left[ {10000 \times {{\left( {1 + \frac{{10}}{{100}}} \right)}^4}} \right] \cr & = {\text{Rs}}.\left( {10000 \times \frac{{11}}{{10}} \times \frac{{11}}{{10}} \times \frac{{11}}{{10}} \times \frac{{11}}{{10}}} \right) \cr & = {\text{Rs}}.14641 \cr & \therefore {\text{C}}{\text{.I}}{\text{. = Rs}}.\left( {14641 - 10000} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}.\,4641 \cr}

3. A sum of money on compound interest amounts to Rs. 10648 in 3 years and Rs. 9680 in 2 years. The rate of interest per annum is = ?
a) 15%
b) 10%
c) 20%
d) 5%

Explanation: Let the sum be Rs. P and rate of interest be R% per annum. Then,
\eqalign{ & P{\left( {1 + \frac{R}{{100}}} \right)^2} = 9680\,.....\,\left( 1 \right) \cr & P{\left( {1 + \frac{R}{{100}}} \right)^3} = 10648\,.....\,\left( 2 \right) \cr}
On dividing equation (2) by (1) :
\eqalign{ & 1 + \frac{R}{{100}} = \frac{{10648}}{{9680}} \cr & \Rightarrow \frac{R}{{100}} = \frac{{10648}}{{9680}} - 1 \cr & \Rightarrow \frac{R}{{100}} = \frac{{10648 - 9680}}{{9680}} \cr & \Rightarrow \frac{R}{{100}} = \frac{{968}}{{9680}} \cr & \Rightarrow \frac{R}{{100}} = \frac{1}{{10}} \cr & \Rightarrow R = \frac{1}{{10}} \times 100 \cr & \,\,\,\,\,\,\,\,\,\,\,\,\, = 10\% \cr}

4. The principal which will amounts to Rs. 270.40 in 2 years at the rate of 4% per annum compound interest, is = ?
a) Rs. 250
b) Rs. 225
c) Rs. 200
d) Rs. 220

Explanation:
\eqalign{ & 4\% = \frac{1}{{25}} \cr & \,\,\,\,\,\,\,\,\, = \frac{{26 \to {\text{Amount}}}}{{25 \to {\text{Principal}}}} \cr & {\text{Time = 2 years}} \cr & {\text{Principal}}\,\,\,\,\,{\text{Amount}} \cr & \,\,\,\,\,\,\,\,\,{\text{25}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{26}} \cr & \,\,\,\,\,\,\,\,\,{\text{25}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{26}} \cr & \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \cr & \,\,\,\,\,\,\,\,625\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,676 \cr & \,\,\,\, \downarrow \times 0.4\,\,\,\,\,\, \downarrow \times 0.4 \cr & \,\,\,\,\,\,\,\,\,250\,\,\,\,\,\,\,\,\,\,\,\,270.40 \cr & {\text{Hence required principal}} \cr & {\text{ = Rs.250}} \cr}

5. The compound interest on a certain sum of money at a certain rate for 2 years is Rs. 40.80 and the simple interest on the same sum is Rs. 40 at the same rate and for the same time. The rate of interest is = ?
a) 5% per annum
b) 3% per annum
c) 4% per annum
d) 2% per annum

Explanation: Difference in CI and SI for 2 years
\eqalign{ & = \left( {40.80 - 40} \right) \cr & = {\text{Rs 0}}{\text{.80}} \cr & {\text{SI for first year }} \cr & {\text{ = }}\frac{{40}}{2} = {\text{Rs}}{\text{.}}\,20 \cr & {\text{Required Rate }}\% \cr & {\text{ = }}\frac{{0.80}}{{20}} \times 100 = 4\% \cr}

6. In how many years will Rs. 2000 amounts to Rs. 2420 at 10% per annum compound interest?
a) 3 years
b) $$2\frac{1}{2}$$ years
c) $$1\frac{1}{2}$$ years
d) 2 years

Explanation:
\eqalign{ & {\text{Principal = Rs. 2000}} \cr & {\text{Amount = Rs. 2420}} \cr & {\text{Rate = 10% }} \cr & {\text{By using formula,}} \cr & \Rightarrow 2420 = 2000{\left( {1 + \frac{{10}}{{100}}} \right)^n} \cr & \Rightarrow \frac{{2420}}{{2000}} = {\left( {1 + \frac{{10}}{{100}}} \right)^n} \cr & \Rightarrow \frac{{121}}{{100}} = {\left( {\frac{{11}}{{10}}} \right)^n} \cr & \Rightarrow {\left( {\frac{{11}}{{10}}} \right)^2} = {\left( {\frac{{11}}{{10}}} \right)^n} \cr & \Rightarrow n = 2 \cr & {\text{Hence,}} \cr & {\text{required time = 2 years}} \cr}

7. The compound interest on Rs.2800 for 18 months at 10% p.a is = ?
a) Rs. 441.35
b) Rs. 436.75
c) Rs. 434
d) Rs. 420

Explanation:
\eqalign{ & {\text{Given,}}\,{\text{Principal}},\,P = Rs.\,2800 \cr & {\text{Compound}}\,{\text{rate}},\,R = 10\% \,{\text{per}}\,{\text{annum}} \cr & = \frac{{10}}{2} = 5\% \,{\text{half - yearly}} \cr & {\text{Amount}} \cr & = {\text{Rs}}{\text{.}}\left[ {2800 \times \left( {1 + \frac{{10}}{{100}}} \right)\left( {1 + \frac{{5}}{{100}}} \right)} \right] \cr & = {\text{Rs.}}\left( {2800 \times \frac{{11}}{{10}} \times \frac{{21}}{{20}}} \right) \cr & = {\text{Rs. }}3234 \cr & \therefore {\text{C}}{\text{.I}}{\text{. = Rs.}}\left( {3234 - 2800} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs. }}434 \cr}

8. If the rate of interest be 4% per annum for first year, 5% per annum for second year and 6% per annum for third year, then the compound interest of Rs.10000 for three years will be ?
a) Rs. 1575.20
b) Rs. 1625.80
c) Rs. 1600
d) Rs. 2000

Explanation:
\eqalign{ & {\text{ = Rs}}.10000\left[ {\left( {1 + \frac{4}{{100}}} \right)\left( {1 + \frac{5}{{100}}} \right)\left( {1 + \frac{6}{{100}}} \right)} \right] \cr & = {\text{Rs}}.\left( {10000 \times \frac{{26}}{{25}} \times \frac{{21}}{{20}} \times \frac{{53}}{{50}}} \right) \cr & = {\text{Rs}}.\left( {\frac{{57876}}{5}} \right) = {\text{Rs}}.11575.20 \cr & {\text{C}}{\text{.I}}{\text{. = Rs}}{\text{.}}\left( {11575.20 - 10000} \right) \cr & \,\,\,\,\,\,\,\,\, = {\text{Rs}}.1575.20 \cr}

9. A bank offers 5% compound interest calculated on half yearly basis. A customer deposits Rs.1600 each on 1st January and 1st July of a year. At the end of the year, the amount he would have gained by way of interest is = ?
a) Rs. 120
b) Rs. 121
c) Rs. 122
d) Rs. 123

Explanation:
\eqalign{ & {\text{ = Rs}}.\left[ {1600 \times {{\left( {1 + \frac{5}{{2 \times 100}}} \right)}^2} + 1600 \times \left( {1 + \frac{5}{{2 \times 100}}} \right)} \right] \cr & {\text{ = Rs}}.\left[ {1600 \times \frac{{41}}{{40}} \times \frac{{41}}{{40}} + 1600 \times \frac{{41}}{{40}}} \right] \cr & {\text{ = Rs}}.\left[ {1600 \times \frac{{41}}{{40}}\left( {\frac{{41}}{{40}} + 1} \right)} \right] \cr & {\text{ = Rs}}.\left( {\frac{{1600 \times 41 \times 81}}{{40 \times 40}}} \right) \cr & {\text{ = Rs}}.\,3321 \cr & \therefore {\text{C}}{\text{.I}}{\text{. = Rs}}.\left( {3321 - 3200} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}.\,121 \cr}

10. The compound interest on Rs 16000 for 9 months at 20% per annum, interest being compounded quarterly, is = ?
a) Rs. 2520
b) Rs. 2518
c) Rs. 2522
d) Rs. 2524

$$\therefore R = \frac{{20}}{4} = 5\%$$
\eqalign{ & \therefore C.I. = P\left[ {{{\left( {1 + \frac{R}{{100}}} \right)}^T} - 1} \right] \cr & = 16000\left[ {{{\left( {1 + \frac{5}{{100}}} \right)}^3} - 1} \right] \cr & = 16000\left[ {{{\left( {\frac{{21}}{{20}}} \right)}^3} - 1} \right] \cr & = 16000\left( {\frac{{9261 - 8000}}{{8000}}} \right) \cr & = 16000 \times \frac{{1261}}{{8000}} \cr & = {\text{Rs}}{\text{.}}\,\,2522 \cr}