One pipe can fill a tank four times as fast as another pipe. If together the two pipes can fill the tank in 30 minutes, then the slower pipe alone will be able to fill the tank in?

Question: One pipe can fill a tank four times as fast as another pipe. If together the two pipes can fill the tank in 30 minutes, then the slower pipe alone will be able to fill the tank in?

Solution:
Let,
the slower pipe alone fill the tank in x minutes. Then, Faster pipe alone will fill it in 4/x minutes.

ATQ,
(1/x) + (4/x) = 1/30
⇒ 5/x = 1/30
∴ x = 150
The slower pipe alone fill the tank in 150 minutes.