Question

Pipe A can fill a tank in 4 hours while Pipe B can empty the same tank completely in 5 hours. If both the pipes are working simultaneously, then determine the time in which an empty tank will be completely filled.

• A. 6
• B. 9
• C. 20
• D. 13

Answer 1

The correct option is C 20

Pipe A can fill a tank completely in 4 hours, Therefore in 1 hour, it can fill $\frac{100}{4}=25\%$ of the tank.

Pipe B can empty the tank in 5 hours, Therefore in 1 hour, it can empty $\frac{100}{5}=20\%$ of the tank.

So, when both are simultaneously working, then in 1 hour $5\%$ of the tank will be filled.

For filling the tank completely, it will take $\frac{100}{5}$ = 20 hours.

2nd method:- $\left(\frac{1}{4}\right)-\left(\frac{1}{5}\right)=\frac{1}{20}$ . Thus tank will fill in 20 days.