A pipe can fill a water tank three times as fast as another pipe. If both are used together, these can fill the tank in 36 minutes. How much time will be taken by the slower pipe: alone to fill the tank?

Correct option is C

Let the slower pipe fill the tank in x minutes.
Then, the faster pipe can fill the same tank in $$\frac x3$$​ minutes.
When both pipes are used together, they can fill the tank in 36 minutes. So their combined rate of filling the tank is = $$\frac1{36}$$​ tank per minute
The slower pipe fills the tank at a rate of $$\frac1x$$​ tank per minute.
The faster pipe fills the tank at a rate of $$\frac{1}{(x/3)} = \frac{3}x$$​ tank per minute.
​$$\frac1x + \frac3x =\frac 1{36}$$​​

​$$\frac4x = \frac1{36}$$​

So, x = 144.