The simple interest as a sum of money is $\frac{4}{5}$ of the principal. Find the rate of Simple interest if both the rate of interest and time are numerically equal.

$4\sqrt{5}$% P.A.

S.I. = $\frac{4}{5}$ of the Principal

P : S.I. = 5 : 4

S.I. = \(\frac{P\;R\;T}{100}\)

4 = \(\frac{5\;R\;T}{100}\)

R× T = 80

Here R = T (given), therefore,

R² = 80

R = 4 \(\sqrt {5}\)