1.  A and B can do a piece of work in 18 days, B and C in 24 days, A and C in 36 days. In what time can they do it all working together ?
A. 12 days
B. 13 days
C. 16 days
D. 26 days

Answer: Option C

Explanation:

(A + B)'s 1 day's work = 1/18

(B + C)'s 1 day's work = 1/24

(A + C)'s 1 day's work = 1/36

Adding 2 (A + B + C)'s 1 day's work = (1/18 + 1/24 + 1/36) = 1/8

∴ (A + B + C)'s day's work = 1/16

Hence, all working together can finish it in 16 days.