SSC CHSL 10 Jan 2017 Shift 3 Paper

Let total work to be done = 24 units
P can complete the work alone in 24 days
⇒ P's efficiency = $\frac{24}{24}$= 1unit/day
Q can complete the work alone in 12 days
⇒ Q's efficiency $\frac{24}{12}$ = 2 units/day
(P + Q)'s 1 day's work = 2 + 1 = 3 units/day
⇒ 50% of the work = $\frac{50}{100}\times 24$ = units
∴ Time taken by them together to complete 50% of the work =123 = 4 days