X can do a work in 15 days and Y in 20 days. If they work on it together for 4 days, then the fraction of the work that is left is :
Answer: D
X's 1 day's work = 1/15. Y's 1 day's work = 1/20. Together, their 1 day's work = \(1/15 + 1/20 = (4+3)/60 = 7/60\). Work done in 4 days = \(4 \times (7/60) = 28/60 = 7/15\). Remaining work = \(1 - 7/15 = 8/15\).