A person borrowed some money on simple interest. After 4 years, he returned 9/5 of the money to the lender. What was the rate of interest?

Given: ​

Amount returned to the lender = $$\frac{9}{5}$$ of the money

Time = 4 years

Formula Used: ​

Simple interest, S.I = Amount + Principal ( Sum) 

S.I = $$\frac{ P \times R \times T }{100}$$ 

where, P = Principal , R = rate of interest, T = time 

​$$\therefore$$ the rate of interest is 20% .​