A wall of 100 meters can be built by 7 men or 10 women in 10 days. How many days will 14 men and 20 women take to build a wall of 600 metres ?

Answer: Option A

Let the required number of days be x
7 men = 10 women
(14 men and 20 women) = (20 + 20) women = 40 women
More length, More days (Direct proportion)
More women, Less days (Indirect proportion)
$$\begin{array}{c}\text{Length 100}:600\text{hfill}\\ \text{Women 40}:10\text{hfill}\end{array}\}::10:x$$
$$\begin{array}{rl}& \therefore 100\times 40\times x=600\times 10\times 10\\ & \iff x=\frac{\left(600\times 10\times 10\right)}{\left(100\times 40\right)}\\ & \iff x=15\end{array}$$