$A$ and $B$ can complete a task in $30$ days when working together. After $A$ and $B$ have been working together for $11$ days, $B$ is called away and $A$, himself completes the task in the next $28$ days. Had $A$ been working alone, the number of days taken by him to complete the task would have been:
Answer is B
$A+B = \frac{1}{30}$
11 days (A+B) work together + 28 days A alone worked = 1
11*$\frac{1}{30}$ + 28 * A = 1
On solving, we will get A’s 1 day work is $\frac{19}{840}$
No: of days taken by A to complete the work alone is $\frac{840}{19}$