Question

If 15 men can reap a field in 28 days; then 5 men can reap the same field in days.

Answer 1

Note that as the number of men available for work increases, the time needed to complete the task decreases, and vice-versa. Thus, this is a case of inverse proportion.
The number of days taken by 15 men to reap a field = 28
The number of days taken by 1 man to reap a field = 28 x 15
The number of days taken by 5 men to reap a field = $\frac{28\times 15}{5}$ = 84 days