A sum of ₹5,000 amountsto ₹7,200 in 8 years at a certain rate per cent p.a, interest compounded yearly. What will be the compound interest on a sum of ₹6,550 in 4 years at the same rate of interest?

From the given question $$\dfrac {7200}{5000} =(1 + \dfrac {r}{100})^8 $$

$$\Rightarrow \dfrac {36}{25} = (1 + \dfrac{r}{100})^8 $$

$$\Rightarrow (\dfrac {6}{5})^2 = [(1+ \dfrac{r}{100})^4]^2$$

$$\Rightarrow (1+\dfrac {r}{}100)^4 = \dfrac {6}{5}$$

it will be used in 2nd condition 

A = $$6550 (1+\dfrac{r}{100})^4 $$

$$\Rightarrow 6550 \times \dfrac{6}{5}$$

$$\Rightarrow 7860 $$

we know that compound intrest (C I) = A - p 

compound intrest = 7860 - 6550 = 1310 Rs Ans