A godown has to be stacked with 6,480 cartons. Each loader stacks 12 cartons per hour of work. If 15 loaders work 9 hours a day, how many days does the stacking take?

A day's output is 15 loaders × 9 hours × 12 cartons = 1,620 cartons, so 6,480 ÷ 1,620 = 4 days. 36 divides 6,480 by $15\times 12=180$, which gives the number of working hours, not days, because the 9-hour day has been left out. 48 divides by $9\times 15=135$, dropping the 12 cartons an hour. 60 divides by $9\times 12=108$, dropping the 15 loaders.