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