A and B working together they can complete in 8 days and A alone can complete that work in 12 days then B alone can do that work in

Work done by A and B together $=\text{A}+\text{B}=8\text{days}$

Work done by A alone $=12\text{\hspace{0.17em}days}$

Work done by B $=?$

Together 1 day work $=\frac{1}{8}$ 

A alone 1 day work $=\frac{1}{12}$

$\Rightarrow \frac{1}{8}=\frac{1}{12}+\frac{1}{x}\Rightarrow \frac{1}{x}=\frac{1}{8}-\frac{1}{12}$

$\frac{1}{x}=\frac{3-2}{24}$

$\frac{1}{x}=\frac{1}{24}$

$x=24\text{\hspace{0.17em}days}$