(c) 5%.
Let the rate of interest p.a. be x %
Then, interest on Rs 6000 after 2 years
= \(\frac{2\times6000\times{x}}{100}\) = 120x = I1
After withdrawing Rs 4000, Rs 2000 remained in the account.
∴ Interest on Rs 2000 for 3 years = \(\frac{3\times2000\times{x}}{100}\) = 60x = I2 .
Given, I1 + I2 = 900
⇒ 120x + 60x = 900 ⇒ 180x = 900 ⇒ x = 5%.