The simple interest on a sum of money is four times the principal, the rate per cent per annum and time in years are equal. Find the rate per cent per annum.

Correct option is D

Let the rate = r% and time = r% 

and Principal = P 

​$$S.I. = \frac{P\times r\times r}{100}\\4P = \frac{P\times r\times r}{100}\\r= 20{\%}$$​​