A and B can do a work in 21 and 28 days respectively. They begin the work together but A left after some days. B completed remaining in 21 days. When did A leaves from the work?

1 day work of $A=\frac{1}{21}$

1 day work of $B=\frac{1}{28}$

21 days work of $B=21\times \frac{1}{28}=\frac{21}{28}=\frac{3}{4}$

1 day work of A & B $=\frac{1}{21}+\frac{1}{28}\Rightarrow \frac{4+3}{84}=\frac{7}{84}=\frac{1}{12}$

The part of work did by both A and B together did =1-$\frac{3}{4}$=$\frac{1}{4}$

let A and B completed $\frac{1}{4} part of the work in$x days

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

x=3 days