10 men working 8 hours day can do a work in 30 days. In how many days will 12 men working 4 hours a day take do the same work?

50 days

Formula to be used here,

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

where, M = No. of men, D = No. of days, H = No. of hours,

⇒ Putting in the values,

⇒ 10 x 30 x 8 = 12 x \( { D}_{2 } \) x 4

⇒ \( { D}_{2 } \) = 50 days.