Interest on Rs 3000 at 5 % p.a. = (3000 * 5 * 1) / 100 = Rs 150.

Let his additional investment at 8 % = x

Interest on Rs x at 8 % p.a. = (x * 8 * 1) / 100 = 2x / 25

To earn 6 % p.a. for the total , Interest = ( (3000+x) * 6 * 1 ) / 100

150 + (2x / 25) = ( (3000+x) * 6 * 1 ) / 100

15000 + 8x = (3000+x) * 6

x = Rs 1500

**Option C is the Correct Answer.**