Working alone, pump A can empty a pool in 3 hours...

Hello LUANDATO.

Let's take a look at your question.

Working alone, pump A can empty a pool in 3 hours, it implies that pump A wor at a rate 1/3 pool/hour.
Working alone, pump B can empty a pool in 2 hours, it implies that pump A wor at a rate 1/2 pool/hour.

Working together the rate will be equal to $$\frac{1}{3}+\frac{1}{2}=\frac{5}{6}\ \text{pool per hour}$$. Now we have the following: $$\frac{5\ }{6\ }\ \frac{pool}{60\ \min}=1\ \frac{pool}{x}\ \Leftrightarrow\ x=72\ \min.$$ This is why the correct answer is the option [spoiler]A=72[/spoiler].

I hope this answer may help you.

I'm available if you'd like a follow-up.

Regards.