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 ?

$$\eqalign{ & {\text{3 men's 1 day's work}} = \frac{1}{{20}} \cr & {\text{5 boy's 1 day's work}} = \frac{1}{{30}} \cr & \left( {{\text{3 men}} + {\text{5 boy}}} \right){\text{'s 1 day's work}} \cr & = \left( {\frac{1}{{20}} + \frac{1}{{30}}} \right) \cr & = \frac{5}{{60}} \cr & = \frac{1}{{12}} \cr} $$
∴ 3 men and 5 boys will complete the work in 12 days.