SSC CHSL 7 Mar 2018 Shift 3 Paper

18 boys can complete the work in 24 days.
⇒ Total work = $18\times 24=432$ MD (mandays)
Now, 6 boys leave after working for 12 days, ⇒ Work done in these 12 days = $18\times 12=216$ MD
Remaining work = $432-216=216$ MD
After 6 boys left, work done per day will be MD
⇒ Time taken = $\frac{216}{12}=18$ days