A can do a piece of work in 12 days, and B in 15 days. They work together for 5 days and then B left. The days taken A to finish the remaining work is :
[A]3 days
[B]5 days
[C]10 days
[D]121 days

3 days
Work done by A and B in 5 days
= 5\left ( \frac{1}{12}+\frac{1}{15} \right ) = 5\left ( \frac{5+4}{60} \right )
= 5\times \frac{9}{60} = \frac{9}{12} = \frac{3}{4}
Remaining work = 1-\frac{3}{4} = \frac{1}{4}
∴ Time taken by A = \frac{1}{4}\times 12 = 3\ days
Hence option [A] is correct answer.