One pipe can fill a tank four times as fast as another pipe. If together the two pipes can fill the tank in 36 minutes, then the slower pipe alone will be able to fill the tank in -

Suppose the slower pipe alone can fill the tank in x minutes.
Then the faster pipe can fill the tank in x/4 minutes.

Part filled by the slower pipe in 1 minute = 1/x
Part filled by the faster pipe in 1 minute = 4/x
Part filled by both the pipes in 1 minute = 1/x + 4/x

Given that both the pipes together can fill the tank in 36 minutes.
Part filled by both the pipes in 1 minute = 1/36

According to the question,
1/x + 4/x = 1/36
5/x = 1/36
x = 180