Ten men begin to do a work. But after some days, four of them left the job. As a result, the job that could have been completed in 40 days is completed in 50 days. How many days after the commencement of the work did the four men leave?

25

Formulae to be used here

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

10 men could have completed the work in 40 days but 4 of the left some days before, so 6 men completed that work in 50 days,

so this is how we put it in formula,

10 x (40-D) = 6 x 50D

solving it we get, 

D = 25 days. 

Therefore, those 4 men left 25 days after the commencement of the work.