Question

Two pipes A and B can fill a cistern in 20 min and 25 min respectively. Both the pipes are opened together, but at the end of 5 min the first is turned off. How long does it take to fill the cistern ?

• A. 16.75 min
• B. 17.75 min
• C. 18.75 min
• D. 19.75 min

Answer 1

The correct option is C 18.75 min

Let it take x minutes to fill the cistern then work done by (Pipe A in 5 min + Pipe B in x min) $=1$
$\Rightarrow \frac{5}{20}+\frac{x}{25}=1$

$\Rightarrow \frac{25+4x}{100}=1$

$\Rightarrow 25+4x=100$

$\Rightarrow 4x=75$

$\Rightarrow x=18.75$ min