A and B can do a work in 12 days and 15 days respectively. Together start the work after 5days B left from the work and A completes the remaining work. How long will A alone complete the remaining?

1 day work of$A=\frac{1}{12}$

1 day work of $B=\frac{1}{15}$

$\begin{array}{l}A+B=\frac{1}{12}+\frac{1}{15}\\ \Rightarrow \frac{5+4}{60}=\frac{9}{60}=\frac{3}{20}\end{array}$

They worked together for 5 days in 5 days they can finish the work of $\Rightarrow 5\times \frac{3}{20}=\frac{3}{4}$

$\frac{3}{4}+\frac{1}{12}x=1$

$\frac{1}{12}x=\frac{1}{4}$

$x=3$

A alone completes the remaining work in 3 days