Correct Answer - Option 3 : 40 hr
Given:
Pipe A can fill the tank alone in 10 hours
Pipe B can fill the tank alone in 20 hours
Pipe C can empty the tank alone in 8 hours
Concept used:
Let, Pipe A can fill a tank in x hours
⇒ Pipe A can fill a tank in 1 hour = 1/x
Calculation:
Pipe A can fill in 1 hour = 1/10
Pipe B can fill in 1 hour = 1/20
Pipe C can empty in 1 hour = 1/8
The total part of the tank filled in 1 hour,
⇒ (1/10) + (1/20) - (1/8)
⇒ (4 + 2 - 5)/40
⇒ 1/40
∴ The tank will be filled in 40 hours.