40 men can complete a piece of work in 18 days. Eight days after they started working together, 10 more men joined them. How many days will they now take to complete the remaining work ?

Let D days required to complete the remaining work
$${40_{{\text{men}}}} \times {18_{{\text{days}}}}$$    = $$\left( {{{40}_{{\text{men}}}} \times {8_{{\text{days}}}}} \right)$$    + $$\left( {{{50}_{{\text{men}}}} \times {\text{D}}} \right)$$
$$\eqalign{ & \Rightarrow 720 - 320 = 50{\text{D}} \cr & \Rightarrow {\text{D}} = 8 \cr} $$