A sum of money becomes \(\frac{39}{12}\) times of itself in 30 years at the rate of r% p.a. simple interest. Find the value of r.

7.5%

        Principal     :        Amount        Simple Interest

            12          :           39         ⇒           27   (in 30 years)

Simple interest per year = \(\frac{27}{30}\) = \(\frac{9}{10}\)

Principal = 12

Rate of Interest (r) = \(\frac{9/10}{12}\) × 100% = 7.5 %