Question

Fifteen workers take 8 hours to build a wall. How many hours will 12 workers need to build the same wall?

Answer 1

As the number of workers increases, the number of hours decreases. The number of workers and number of hours are in inverse proportion. The product of the number of workers and the number of hours needed to build the wall is constant. Let us use the variable $x$ to solve this problem.
Suppose, 12 workers take $x$ hours. 15 workers take 8 hours.
$12\times x=15\times 8$
$\therefore 12x=120$
12 workers take $x$ hours.
$\therefore x=10$
Thus, 12 workers will take 10 hours to build the wall.