1.  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. 4
B. 8
C. 5
D. 9

Answer: Option D

Explanation:

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)

= 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