According to the question,

In 10 days (P+Q) complete$=\frac{1}{30}\times 10=\frac{1}{3}$^th of the work

Remaining work$=\left(1-\frac{1}{3}\right)=\frac{2}{3}$

$\frac{2}{3}$ ^th of the work is done by P in 20 days.

So, whole work can be completed by P in$=20\times \frac{3}{2}=30$days.

Hence option A is the correct.