1.  A can do a piece of work in 30 days while B can do it in 40 days. In how many days can A and B working together do it ?
A. 70 days
B. 423/4 days
C. 271/7 days
D. 171/7 days

Answer: Option D

Explanation:

(A + B)'s 1 day's work = (1/30 + 1/40) = 7/120

∴ Time taken by both to finish the work = 120/7days = 171/7 days