JAMB Mathematics 2020 Paper

Concept:Use the rate of one girl as the basic unit to compare the rates of boys and girls.Explanation:Let the rate of one girl be $g$ field per hour.Then the rate of one boy is $\frac{5}{4}g$.Rate of 4 boys and 10 girls is:$4\left(\frac{5}{4}g\right) + 10g = 5g + 10g = 15g$In 5 hours, the total work done is:$15g \times 5 = 75g$To finish the same work in 3 hours, the required rate is:$\frac{75g}{3} = 25g$Let $n$ be the number of boys needed.Then $n \times \frac{5}{4}g = 25g$.So $n = 25 \times \frac{4}{5} = 20$.Answer:20 boys are needed.Option D.