1.  P can do piece of work in 12 days, while Q alone can finish it in 8 days. With the help of R, they can finish the work in 4 days. How long will R take to finish the work alone ?
A. 25 days
B. 34 days
C. 14 days
D. 24 days
E. None of the above

Answer: Option D

Explanation:

P's 1 day's work = 1/12

Q's 1 day's work = 1/8

(P + Q)'s 1 day's work = (1/12) + (1/8)

= (2 + 3)/24

= 5/24


(P + Q + R)'s 1 day's work = 1/4

∴ R's 1 day's work = (1/4) - (5/24) = (6 - 5)/24 = 1/24

∴ R will finish the work alone in 24 days.

∴ Required time = 24 days