The question states that 3 men or 15 boys can finish the work in 30 days.
Both groups take the same time (30 days), so they have the same working power.
Therefore: 3 men = 15 boys
Dividing both sides by 3:
1 man = 5 boys
Converting the 5 boys into men using this relationship:
5 boys = 1 man
The total working power of 2 men and 5 boys:
2 men + 5 boys = 2 men + 1 man
Total = 3 men
3 men can finish the work in 30 days.
Since 2 men + 5 boys = 3 men, the time needed is 30 days.