Management Aptitude Test 2015 Paper

(b) Given, pipe A can fill a tank in 30 min,
B and C can fill the tank In 90 min
and A and B can fill the tank in 20 min.
Now, pipe B alone can fill the tank in
= $\frac{1}{\mathit{A}+\mathit{B}}-\frac{1}{\mathit{A}}$
= $\frac{1}{20}-\frac{1}{30}$
= $\frac{3-2}{60}=\frac{1}{60}$
And pipe C alone can empty the tank in
= $\frac{1}{\mathit{B}+\mathit{C}}-\frac{1}{\mathit{B}}$
= $\frac{1}{90}-\frac{1}{60}$
= $\frac{2-3}{180}=-\frac{1}{180}$
Here, negative sign means empty.
So, the pipe C can empty the tank in 180 min.