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?

10 days

Formula to be used here,

\( { M}_{1 } \) x \( { D}_{1 } \)  = \( { M}_{2 } \) x \( { D}_{2 } \)

⇒ where, M = No. of Men, D = No. of Days,

⇒ Putting values in the formula,

⇒ 25 x 12 = 30 x \( { D}_{2 } \)

⇒ \( { D}_{2 } \) = 10 days.