Given, 4 men can complete a work in = 15 days
5 women can complete the same work in = 12 days
3 boys can complete the same work in = 20 days
Formula: Total work = Efficiency × Time taken
Calculation: Total work = 60
Efficiency of 4 men = 4 units/day
⇒ Efficiency of 1 man
== 1 units/day
Efficiency of 5 women = 5 units/day
⇒ Efficiency of 1 woman
= = 1 unit/day
Efficiency of 3 boys = 3 units/day
⇒ Efficiency of 1 boys
= = 1 unit/day
Time taken by 1 man, 1 woman and 1 boy together to complete the work
⇒
==20 days
∴ The required time is 20 days.