Let T be the total number of days to complete the project.
A can do $\frac{1}{20}$ of work in one day and B can do $\frac{1}{30}$ in one day and to gather they can for$\frac{1}{20} + \frac{1}{30} = \frac{1}{12}$ of the work.

So if A quit 10 days before the project is over, then T-10 days both worked and 10 days B alone worked to complete 1 task.
$1 = \frac{(T-10)}{12} + \frac{10}{30}$
Solving to T, will be T = 18 days.
Answer is A) 18