Given: Rs. 600 become Rs. 720 in 4 years Concept Used: Interest = Amount - Principal If the principal is p, time is t and the rate of interest is r then the simple interest is ptr/100 Calculation: Rs. 600 become Rs. 720 in 4 years that means Principal (p) = 600 Amount (A) = 720 Time (t) = 4 year Let, the rate of interest be r% Total interest =720−600 ⇒120 (600×4×r)}100=120 ⇒24r=120 ⇒r=5 ∴ The rate of simple interest per annum is 5%