30 labourers working 7 hours a day can finish a piece of work in 18 days. If the labourers work 6 hours a day, then the number of labourers to finish the same piece of work in 30 days, will be ?

Answer: Option B

Let the required number of labourers be x
Less working hour/day, More labours (Indirect proportion)
More days, Less labourers (Indirect proportion)
$$\begin{array}{c}\text{Working hours/day 6}:7\text{hfill}\\ \text{Days 30}:18\text{hfill}\end{array}\}::30:x$$
$$\begin{array}{rl}& \therefore 6\times 30\times x=7\times 18\times 30\\ & \iff 6x=126\\ & \iff x=21\end{array}$$