Indian Institute of Foreign Trade 2011 Solved Paper

It is given that a man is able to perform the work equal to the same done by 2 boys. Hence, we can assume that In Bilaspur village 21=(12+18/2) men completed construction of a primary health center in 60 days, by working for 7.5 hours a day.
Hence, total men hours required to completed the primary health center in Bilaspur $=7.5\times 21\times 60=9450.$
Let us assume that 'n' be the number of boys required to help 21 men to complete the work in Harigarh in 50 days by working 9 hours a day.
Therefore, we can say that
$\Rightarrow 2\times 9450=50\times 9\times \left(21+\frac{\mathit{t}}{2}\right)$
$\Rightarrow 42=21+\frac{\mathit{t}}{2}$
$\Rightarrow \mathit{t}=42$
Hence, option D is the correct answer.