25 men can complete a task in 16 days. Four days after they started working, 5 more men, with equal workmanship, joined them. How many days will be needed by all to complete the remaining task?

Solution (By Examveda Team)

$$\eqalign{ & 25 \times 16 = 25 \times 4 + 30 \times x \cr & \left( {{M_1} \times {D_1}} \right) = \left( {{M_1}{D_1} \times {M_2}{D_2}} \right) \cr & 400 = 100 + 30x \cr & 300 = 30x \cr & x = 10{\text{ days}} \cr} $$