If the compound interest for the 3rd and 4th year on a certain principal is Rs.20 and Rs.35 respectively, what is the rate of interest (in %)?

75%

Compound interest for the 3rd year = 20

Compound interest for the 4th year = 35

R% = \(\frac{ (CI \; for \; 4th \; year) - (CI \; for \; 3rd \; year) }{(CI \; for \; 3rd \; year)}\)

R% = \(\frac{35 - 20}{20}\) × 100 = 75%