SSC CHSL 4 Mar 2018 Shift 1 Paper

Let capacity of tank = L.C.M. (20,35) = 140 litres
Pipe X can fill the tank in 20 hours, ⇒ X's efficiency = $\frac{140}{20}=7$ l/hr
Similarly, Y's efficiency = $\frac{140}{35}=4$ l/hr
Pipes Y and X are alternatively opened, ⇒ Tank filled in 2 hours = $4+7=11$ litres
Multiplying both sides by 12, we get ⇒ Tank filled in hours = 132 litres
Now, its Y's turn to be opened, it will 4 more litres, Now, its Y's turn to be opened, it will 4 more litres, ⇒ In 25 hours = litres
Thus, the remaining tank is filled by pipe X in $\frac{4}{7}$ hours
⇒ Total time = $25+\frac{4}{7}=\frac{179}{7}$ hours