WAEC Mathematics 2012 Paper

Concept:Simple interest is earned only on the original principal, so first find the interest by subtracting the principal from the amount.Explanation:Given: Principal $P = \text{N}2{,}500$, Amount $A = \text{N}3{,}500$, Time $T = 4$ years.Interest, $I = A - P = \text{N}3{,}500 - \text{N}2{,}500 = \text{N}1{,}000$.Use the simple interest formula: $I = \dfrac{PRT}{100}$.Substitute the known values: $1{,}000 = \dfrac{2{,}500 \times R \times 4}{100}$.Simplify the right-hand side: $1{,}000 = 100R$.Therefore, $R = \dfrac{1{,}000}{100} = 10$.Answer:Rate of interest = $10\%$ per annum.Correct option: D. $10\%$.