Let the rate of simple interest be x%.

Then, interest on Rs. 800 in 2 years = (800*x*2)/100 = 16x Interest on Rs. 400 in 2 years = (400*x*2)/100 = 8x

Given, the interest on Rs. 800 in 2 years is more than the interest on Rs. 400 by Rs. 40 in 2 years.

So, we have: 16x – 8x = 40 8x = 40 x = 5

Therefore, the rate of simple interest is 5%.