Let the principal P = Rs 100
Given it is doubled after 4 years
i.e. Time n = 4 years
After 4 years A = Rs 200
∴ A = P + I
A – P = I
200 – 100 = I
After 4 years interest I = 100
I = \(\frac{pnr}{100}\)
⇒ 100 = \(\frac{100\times4\times\,r}{100}\)
4r = 100
⇒ r = 25 %
Rate of interest r = 25 %