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### Question

The compound amount after 3 years on a principal of Rs. x is same as that on a principal of Rs. (621 - x) after 4 years, then what is x if the rate of interest is 7% p.a. compounded yearly?

**A**

Rs. 321.

**B**

Rs. 421.

**C**

Rs. 221.

**D**

Rs. 521.

**Soln.**

**Ans: a**

We have x × $(1 + 7/100)^3$ = (621 - x) × $(1 + 7/100)^4$. Cancelling, we get x = (621 - x) × (1 + 7/100). Simplifying, x = ${621 × (100 + 7)}/(200 + 7)$, which gives x = Rs. 321.