A and B can do a work in 20 days and 12 days respectively. B worked at it for 9 days and then leaves. In how many days A alone can finish the remaining.

1 day work of $A=\frac{1}{20}$

1 day work of $B=\frac{1}{12}$

9 days work of B, $9\times \frac{1}{12}=\frac{3}{4}$

Remaining work $=1-\frac{3}{4}=\frac{1}{4}$

A alone  can finish the work in

$\frac{1}{20}x=\frac{1}{4}$

A alone can finish the remaining work in 5 days