Question

Two pipes A and B can fill a tank in $4$ hours and $5$ hours respectively. If they are turned up alternatively for one hour each, the time taken to fill the tank is

• A. $2hrs15min$
• B. $4hrs24min$
• C. $5hrs$
• D. $3hrs$

Answer 1

The correct option is B $4hrs24min$

Tank filled in $2hours=\frac{1}{4}+\frac{1}{5}=\frac{9}{20}$
Tank filled in $4hours=2\times \frac{9}{20}=\frac{9}{10}$.
Now $\frac{1}{10}th$ of the tank filled by A in $(\frac{1}{10}\times 4\times 60)min$. i.e., $24$ min. Total time $=4hrs.24min$.