A job can be done by 3 skilled workmen in 20 days or by 5 boys in 30 days. How many days will they take if they work together?

Question: A job can be done by 3 skilled workmen in 20 days or by 5 boys in 30 days. How many days will they take if they work together?

Solution:
3 men's 1 days work = 1/20 and 5 boy's 1 day's work = 1/30
(3 men + 5 boy)'s 1 day's work = (1/20 + 1/30)
= 5/60 = 1/12

∴ 3 men and 5 boys will complete the work in 12 days.