A man, a woman and a boy can do a piece of work in 6, 9 and 18 days respectively. How many boys must assist one man and one woman to do the work in 1 day ?

Answer: Option D

(1 man + 1 woman)'s 1 day's work
$$\begin{array}{rl}& =\frac{1}{6}+\frac{1}{9}\\ & =\frac{5}{18}\\ & \text{Remaining work}\\ & =\left(1-\frac{5}{18}\right)\\ & =\frac{13}{18}\end{array}$$
Work done by 1 boy in 1 day = $$\frac{1}{18}$$
$$\begin{array}{rl}& \therefore \text{Number of boys required}\\ & =\left(\frac{13}{18}\times 18\right)\\ & =13\end{array}$$