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With a given rate of simple inerest, the ratio of principal and amount for a certain period of time is 4 : 5. After 3 years, with the same rate of interest, the ratio of the principal and amount becomes 5 : 7. The rate of interest is
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- 4%
- 6%
- 5%
- 7%
- 4%
Correct Option: C
Case-I,
Let Principal be 4p and Amount be 5p .
Simple Interest = Amount - Principal = 5p – 4p = p
We know that ,
| SI = | ||
| 100 |
| ∴ p = | |
| 100 |
| ⇒ T = | Years. | |
| R |
Case-II,
Let Principal be 5q and Amount be 7q .
| T = | + 3 = |  |  | Years. | ||
| R | R |
Simple Interest = Amount - Principal = 7q – 5q = 2q
| ∴ 2q = | |
| R × 100 |
⇒ 40 = 25 + 3R
⇒ 3R = 40 – 25 = 15 %
| ⇒ R = | = 5% | |
| 3 |
