DRDO Multi tasking staff Model paper 7

P, Q and R together can complete a work in 20 days
Part of work P,Q and R can complete in 1 day = 1/20
P can complete the same work in 40 days
Part of work P can complete in 1 day = 1/40
Q can complete the same work in 60 days
Part of work Q can complete in 1 day = 1/60
∴ Part of work R clone can do in 1 day = 1/20 – 1/40 – 1/60 = 1/120
∴ R alone can do the work in 120 days