In how many days can ‘Q’ alone complete the work?
I. P, Q and R together can complete the work in 9`(3)/(13)`   days.
II. P and Q together can complete the work in 10 days, Q and R together the work together in 20 days, and P and R together can complete the work in 15 days.

From I. (P + Q + R) can do the work in 93/13 days. Hence, from I alone we cannot get the answer. From II.  (P + Q)’s one day work = 1/10 part of the work……… (i) (Q + R)’s one day work = 1/20 part of the work……….. (ii) (P + R)’s one day work = 1/15 part of the work………… (iii) ∴ 2(P + Q + R)’s one day work = 1/10 + 1/20 +1/15 = 13/60 ∴ (P + Q + R)’s one day work =  13/120………….. (iv) By subtracting (iii) by (iv), we get Q’s one day work. ∴ Q’s one day work = 13/120 - 1/15 = 5/120 = 1/24 Hence, from II alone we can get the answer.