The ratio between principal to amount is 4 : 5 for certain time and on certain rate of simple interest. After three years that ratio becomes 5 : 7. Find rate of interest.

05%

                      Principal   :    Amount

Original      ⇒        4        :        5      ..............(i)

After 3 yrs. ⇒        5        :        7      ..............(ii)

In the case of S.I., principal is always remains same, therefore, to make the principal equal:

(i) × 5 and (ii) × 4

                      Principal   :    Amount

Original      ⇒       20        :       25

After 3 yrs. ⇒       20        :       28

We can see,

Amount increased by 3R in 3 years, it means S.I. received in 3 years is 3R,

Therefore,

S.I. in one year = 1R

Principal = 20R0

Rate of Interest = \(\frac{1}{20}\) × 100% = 5% pa