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?

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