A and B can do a piece of work in 12 days, B and C can do it in 15 days and C and A can do the same work in 20 days. Find the number of days in which A alone can do the same job?

Work done by A and B in one day = 1/12
Work done by B and C in one day = 1/15
Work done by C and A is one day = 1/20
On adding all these,
2(A + B + C)'s work of one day = 1/12 + 1/15 +1/20 = 1/5
⇒ (A + B + C)'s one day work = 1/10
∴ A' s one day work = (A + B + C)'s one day work - (B + C )'s one day work
⇒ A's one day work 1/10 - 1/15 = 1/30
Hence, A can do this work in 30 days.