An automobile tire has two punctures. The first puncture by itself would make the tire flat in 9 minutes. The second puncture by itself would make the tire flat in 6 minutes. How long will it take for both punctures together to make the tire flat? (Assume the air leaks out at a constant rate.)

(A) 18/5 minutes
(B) 4 minutes
(C) 21/4 minutes
(D) 15/2 minutes
(E) 15 minutes
OA is a

which formula will i use here to get the correct answer to this question? can any expert help me here?
Thank you

Hi Roland2rule,
Let's take a look at your question.

The questions states that:
"The first puncture by itself would make the tire flat in 9 minutes. The second puncture by itself would make the tire flat in 6 minutes."

In one minute, the part of the tire made flat by the first puncture = 1/9
In one minute, the part of the tire made flat by the second puncture = 1/6

Now we can find out the part of the tire made flat by both the punctures in one minute by just adding them up.
In one minute, the part of the tire made flat by both the punctures =
$$=\frac{1}{9}+\frac{1}{6}$$
$$=\frac{2+3}{18}$$
$$=\frac{5}{18}$$

Reading it in reverse order, we can now say that both the punctures together will make the tire flat in 18/5 minutes.

Therefore, Option A is correct.

Hope it helps.
I am available if you'd like any follow up.