SSC CHSL 8 Jan 2017 Shift 1 Paper

Let total work to be done = 12 units
P can complete the work alone in 6 days
⇒ P's efficiency = $\frac{12}{6}=2$ units/day
Q can complete the work alone in 12 days
⇒ Q's efficiency = $\frac{12}{12}=1$ unit/day
(P + Q)'s 1 day's work = 2 + 1 = 3 units/day
∴ Time taken by them together to complete 25% of the work = $\frac{25}{100}\times \frac{12}{3}=\frac{1}{4}\times 4=1$ day