With a given rate of simple interest, the ratio of principal and amount for a certain period of time is 4 : 5. After 3 years, with the same rate of interest, the ratio of the principal and amount becomes 5 : 7. The rate of interest is?

5%

             P  :   A                     ⇒       P  :  A

               4 : 5     after 3 years        5 : 7 

Since P remains same

               20 : 25                            20 : 28

28 - 25 = 3 ( in 3 years)

SI in 1 year = \(\frac{3}{3}\) = 1

Rate = \(\frac{1}{20}\) × 100 % = 5%