DRDO Multi tasking staff Model paper 3

Let Pankaj take ‘x’ days to do the work alone
Pankaj’s 1 day work = 1/x
Punit’s 1 day work = 3/x
When they work together, their 1 day work = 1/x + 3/x = 4/x
⇒ 4/x = 1/5
⇒ x = 4 ×5 = 20 days
∴ Pankaj will take 20 days to do the work alone