A can do a work in 15 days and B in 20 days. If they work on it together for 4 days, then the fraction of the work that is left is:

1 दिन का कार्य $= 1/15 + 1/20 = 7/60$। 4 दिनों में किया गया कार्य $= 4 imes 7/60 = 7/15$। शेष कार्य $= 1 - 7/15 = 8/15$।