A sum of ₹3,000 amounts to ₹6,000 in two years at compound interest. The interest for four years is:

Under compound interest, a sum grows by a constant multiplier over equal periods of time.
Given:
Initial Principal ($P$) = ₹3,000
Amount after $2$ years ($A_2$) = ₹6,000

The growth factor for a $2$ -year interval is:

$$k=\frac{A_2}{P}=\frac{6000}{3000}=2$$

This means the amount doubles every $2$ years.
Therefore, the amount after $4$ years ($A_4$) will be:

$$A_4=A_2\times k=6000\times 2=\text{₹}12,000$$

The total interest earned over $4$ years is:

$$\text{Interest}=A_4-P=12000-3000=\text{₹}9,000$$