Question

A large tanker can be filled by two pipes $A$ and $B$ in $60$ min and $40$ min. respectively. How many minutes will it take to fill the tanker from empty state if $B$ is used for half the time and $A$ and $B$ fill it together for the other half ?

• A. $15$ min
• B. $20$ min
• C. $27.5$ min
• D. $30$ min

Answer 1

Answer: (D)

$30$ min

Part fill by (A+B) in 1 minute$=\frac{1}{60}+\frac{1}{40}=\frac{5}{120}=\frac{1}{24}$

Let the tank is filled in x minute then

$\frac{x}{2}\times (\frac{1}{24}+\frac{1}{40})=1$

$\frac{x}{2}\times \frac{1}{15}=1$

$\frac{x}{30}=1$

$x=30min$