A pipe A can fill a tank in 4 hours. There are two outlet pipes, B and C, from the tank which can empty it in 8 and 10 hours respectively. If all the three pipes are opened simultaneously, how long will it take to fill the tank?

40 hours

The correct answer is Option (2) → 40 hours **

$\text{Rate of A}=\frac{1}{4}$

$\text{Rate of B}=\frac{1}{8}$

$\text{Rate of C}=\frac{1}{10}$

$\text{Net rate}=\frac{1}{4}-\frac{1}{8}-\frac{1}{10}$

$=\frac{10}{40}-\frac{5}{40}-\frac{4}{40}$

$=\frac{1}{40}$

The time taken to fill the tank is $\;40\;\text{hours}.$