Time and work Quantitative aptitude question

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

(a) 15/8
(b) 8/15
(c) 17/8
(d) 8/17

Solution

Answer   (b)
Explanation
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Amount of work A can do in 1 day = 1/15

Amount of work B can do in 1 day = 1/20

Amount of work A and B can do in 1 day = 1/15 + 1/20 = 7/60

Amount of work A and B can together do in 4 days = 4 × (7/60) = 7/15

Fraction of work left = 1 – 7/15= 8/15