Time and work Quantitative aptitude question

[Q] A can finish a work in 18 days and B can do the same work in 15 days. B worked for 10 days and left the job. In how many days, A alone can finish the remaining work?
(a) 4
(b) 2
(c) 5
(d) 6

Solution

Answer   (d)
Explanation
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Work done by A in 1 day = 1/18

Work done by B in 1 day = 1/15

Work done by B in 10 days = 10/15 = 2/3

Remaining work = 1 – 2/3 = 1/3

Number of days in which A can finish the remaining work = (1/3) / (1/18) = 6