A alone can do a certain job in 15 days, while B alone can do it in 10 days. A started the work and was joined by B after 5 days. The work lasted for how many days?

A's 5 days' work = (1/15) x 5 = 1/3
Remaining work = 1 - (1/3) = 2/3
∴ (A + B)'s 1 day's work
= (1/15) + (1/10)
= (2 + 3(/30
= 5/30
= 1/6
∴ 2/3 work will be finished by (A + B) in (2/3) x 6 days or in 4 days .
∴ Total time taken = (5 + 4) days = 9 days