Management Aptitude Test 2015 Paper

(d) Since, pipe A takes 3h to fill $\frac{4}{5}$th part of the tank.
So, pipe A can fill the full tank in $\frac{5}{4}$ x 3 i.e. in $\frac{15}{4}$ h.
Let C can empty the full tank in xh.
Then, according to the question,
A's (5 + 3) h work + C’s 5h work = 20%
⇒ $8\times \frac{4}{15}-\frac{1}{\mathit{x}}\times 5=\frac{1}{5}$
⇒ $\frac{5}{\mathit{x}}=\frac{32}{15}-\frac{1}{5}$
⇒ $\frac{5}{\mathit{x}}=\frac{29}{15}$
⇒ $\frac{1}{\mathit{x}}=\frac{29}{75}$
∴ C can empty the tank in $\frac{75}{29}$h
Thus, time taken by C to empty 20% of the tank (as only 20% part of the tank is filled)
= $\frac{75}{29}\times \frac{1}{5}=\frac{15}{29}$h