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

Let the slower pipe alone fill the tank in x minutes. 

Then,faster pipe will fill it in x/3 minutes. 

$=>\frac{1}{x}+\frac{3}{x}=\frac{1}{36} =>\frac{4}{x}=\frac{1}{36} =>x=144mins$