Compound Interest, General Mathematics

Question: The compound interest on Rs. `30000` at `7%` per annum for a certain period is Rs.`4347`
A
`2 years`

B
`2 1/2 years`

C
`3 years`

D
`4 years`

Note: Amount `= Rs.(30000 + 4347) ` `= Rs.3447`. Let `30000 (1 + 7/100)^n` `= 34347` `or (107/100)^n` `= 34347/30000` `= 11449/10000` `= (107/100)` `:. n = 2 years`
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Question: At what rate of compound interest per annum will a sum of Rs.`1200` become Rs.`1348.32` in `2` years ?
A
`7%`

B
`6%`

C
`7.5%`

D
`6.5%`

Note: `1200 xx (1 + R/100)^2 + 1348.32` `or (1 + R/100)^2` `= 134832/120000` `= 11236/10000` `:. (1 + R/100)^2` `= (106/100)^2` `or 1 + R/100` `= 106/100` `or R = 6%`
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Question: A sum of money invested at compound interest amounts to Rs.`800` in `3` years and Rs.`840` in `4` years. What is the rate of interest per annum ?
A
`2%`

B
`4%`

C
`5%`

D
`10%`

Note: Interest on Rs.`800` for `1` year = Rs. (840 - 800)` `= Rs. 40` :. Rate`= (100 xx 40)/(800 xx 1)%` `= 5%`
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Question: A sum of money becomes `8` times of itself in `3` years at compound interest. The rate of interest is :
A
`100%`

B
`8%`

C
`1%`

D
date inadequate

Note: P (1 + R/100)^3 = 8P` `or (1 + R/100)^3` `= 8 = 2^3` `:. 1 + R/100 = 1` `or R = 100`
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Question: The compound interest on Rs. `30000` at `7%` per annum for a certain period is Rs.`4347`