If the compound interest for the 3rd and 4th year on a certain principal is Rs. 26 and Rs. 39 respectively, what is the rate of interest (in %)?

50%

Compound interest for the 3rd year = 26

Compound interest for the 4th year = 39

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

R% = \(\frac{39 - 26}{26}\) × 100 = 50%