Pipe A can fill an empty tank in 8 hours and pipe B can fill it in 12 hours. The tank is empty and both pipes are opened together. After 3 hours pipe A is closed, and pipe B alone fills the rest. How much longer does pipe B take to fill the tank?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4 hours 30 minutes
Find the part filled in the first 3 hours by both pipes; B alone then fills the part left.
A and B in one hour \(\displaystyle =\frac{1}{8}+\frac{1}{12}=\frac{3+2}{24}=\frac{5}{24}\)
In 3 hours \(\displaystyle =3\times\frac{5}{24}=\frac{5}{8}\)
Part left \(\displaystyle =1-\frac{5}{8}=\frac{3}{8}\)
Time for B \(\displaystyle =\frac{3}{8}\times12=\frac{9}{2}=4\frac{1}{2}\) hours
B takes 4 hours 30 minutes more.