A certain sum of money at simple interest amounts to Rs. 1012 in 2 1/ 2 years and to Rs. 1067.20 in 4 years. The rate of interest per annum is?

∵ S.I. for 3/2 years = Rs. (1067.20 - 1012) = Rs. 55.20
⇒ S.I. for 5/2 years = Rs. 55.20 x (2/3) x (5/2) = 92
∴ Sum = Rs. (1012 - 92) = Rs.920
Hence, Rate = (100 x SI)/(P x T) = (100 x 92) / (920 x 5/2 ) = 4%