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A sum of money invested at compound interest amounts to Rs. 800 in 3 years and to Rs. 840 in 4 years. The rate of interest per annum is :
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2 1 % 2 - 4%
- 5%
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6 2 % 3
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Correct Option: C
Let principal be P.
Given that , A1 = Rs. 800 , T1 = 3 years and A2 = Rs. 840 , T2 = 4 years , R1 = R2 = R%
Using the given formula , we have
| A = P |  | 1 + |  | T | |
| 100 |
| ⇒ 800 = P | | 1 + | | 3 | ....(i) | |
| 100 |
and,
| 840 = P | | 1 + | | 4 | ....(ii) | |
| 100 |
On dividing equation (ii) by (i),
| = 1 + | ||
| 800 | 100 |
| ⇒ | = 1 + | ||
| 20 | 100 |
| ⇒ | = | − 1 = | |||
| 100 | 20 | 20 |
| ⇒ R = | × 100 = 5% per annum | |
| 20 |
