If simple interest earned on a certain sum for the 4 years is Rs. 19,200 and compound interest earned in 2 years is Rs. 10,200/-, then find the rate of interest p.a.

12.5%

S.I. is constant for each year

⇒ S.I. in 4 year = Rs.19200/-

⇒ S.I. in 1 year =\(\frac{19200}{4}\) = Rs. 4800/-

⇒ S.I. in 2 year =\(\frac{19200}{2}\) = 9600

⇒ C.I. in 2 years = Rs.10200/-

Difference b/w C.I. and S.I. = 10200 - 9600 = 600

⇒ Rate of Interest = \(\frac{(600) × 100}{4800}\)

⇒ R = 12.5%