NCHMCT JEE 2021 Question Paper with solutions for online practice

Time taken to complete the work by $\mathit{B}=12$ days
$\because \mathit{A}$ work twice as fast as $\mathit{B}$
$\therefore$ Time taken to complete the work by $\mathit{A}=6$ days
Now, one day work of $\mathit{B}=\frac{1}{12}$ and one day work of $\mathit{A}=\frac{1}{6}$
$\therefore$ One day work of $\left(\mathit{A}+\mathit{B}\right)=\frac{1}{6}+\frac{1}{12}=\frac{2+1}{12}=\frac{3}{12}=\frac{1}{4}$
Hence, time taken by $\mathit{A}$ and $\mathit{B}$ together to finish the work in $=4$ days.