Correct Answer - Option 4 : 4%
The Sum of money doubles itself in 25 years.
Simple interest is the interest calculated on the principal portion of the loan or the original contribution to the saving account.
Simple Interest = (P × R × T)/100
S.I = Amount - Sum
Let the sum be 'P' then Amount after 25 years = 2P
S.I = 2P - P = P
P = (P × R × 25)/100
⇒ R = 100/25 = 4%
∴ The rate of interest per annum is 4%.