One pipe works three times as fast as another. The two pipes together can fill a tank in 26 minutes. By working alone, how long will the slower pipe take to fill the tank?

Correct option is D

Given:
The faster pipe works three times as fast as the slower pipe.
Together, they can fill the tank in 26 minutes.
Solution:
Work rate of slower pipe = 1/x
Work rate of faster pipe = 3/x
Combined work rate = 1/26
According to questions,
​$$\begin{aligned}& \frac{1}{x} + \frac{3}{x} = \frac{1}{26} \\& \frac{4}{x} = \frac{1}{26} \\& x = 4 \times 26 \\& x = 104\end{aligned}$$​​
Thus, the slower pipe will take 104 minutes to fill the tank.

Alternate Solution:
Let the efficiency of faster pipe = 3x unit per minutes
And efficiency of slower pipe = x unit per minutes
Total Work = (3x+x)×26 = 104x unit
Time taken by slower pipe to fill the tank =$$\frac{104x}{x} = 104 \text{min.}$$​​