A can do a piece of work in 5 hours, B in 9 hours and C in 15 hours. If C could work with them for 1 hour only the time taken by A and B together to complete the work is?

∵ (1/5 + 1/9 + 1/15) = 17/45 work is finished in 1 hour
∴ Remaining work = 1 - 17/45 = 28/45
⇒ (A + B)'s 1 hour's work = 1/5 + 1/9 = 14/45
14/45 work is done by (A and B) in 1 hour
28/45 work will be done by A and B in (45/14) x (28/45) = 2 hours