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

1/4
1/10
7/15
8/15

Correct answer: 8/15

Solution

Together, A and B complete $\frac{1}{15}+\frac{1}{20}=\frac{7}{60}$ of the work per day. In 4 days they complete $\frac{28}{60}=\frac{7}{15}$, so the remaining work is $1-\frac{7}{15}=\frac{8}{15}$.

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