Calculation: Let the sum invested be Rs. P and the period of investment be X years ⇒ Rate of interest = X% ⇒ Simple interest obtained = Rs. P According to the question, ⇒ P = (P × X × X) /100 ⇒ X2 = 100 ⇒ X = 10 years ⇒ Period of investment = 10 years