Q) 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:   

a) 1/4              b) 1/10               c) 7/15                     d) 8/15

Ans:

A’s 1 day’s work = 1/15

B’s 1 day’s work = 1/20

∴ (A + B)’s 1 day’s work = 1/15 + 1/20 = 7/60

∴ (A + B)’s 4 days work = 4 x 7/60 = 28/60

∴ Balance work = 1 – 28/60 = 32/60 = 8/15

Hence, Answer is [D].

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