Question:
A work-force of 420 men with a contractor can finish a certain piece of work in 9 months. How many extra men must he employ to complete the job in 7 months?
Solution:
Let x be the extra number of men employed to complete the job in 7 months.
Since the number of men hired and the time required to finish the piece of work are in inverse variation, we have :
$420 \times 9=7 x$
$\Rightarrow x=\frac{420 \times 9}{7}$
$=540$
Thus, the number of extra men required to complete the job in 7 months $=540-420=120$