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

Suppose, 12 workers take x hours
As the number of workers increases, the number of hours decreases.
The number of workers and number of hours are in inverse proportion.
$\therefore 12\times x=15\times 8$
$\therefore 12x=120$
$\therefore x=10$
Thus, 12 workers will take 10 hours to build the wall.