SSC CHSL 8 Jan 2017 Shift 3 Paper

Let total work to be done = 24 units
Prabhat has done 1/2 of a job i.e. $\frac{24}{2}=12$ units in 12 days
⇒ Prabhat's efficiency = $\frac{12}{12}=1$ unit/day
Work left = 24 - 12 = 12 units which is done by Santosh in 6 days
⇒ Santosh's efficiency = $\frac{12}{6}=2$ units/day
Prabhat and Santosh 1 day's work = 1 + 2 = 3 units/day
∴ Time taken by them together to compiete the work $=\frac{24}{3}=8$ days