Correct Answer - Option 3 : 6 hrs 40 min
Given
1st pipe can fill the tank = 10 hrs
2nd pipe can fill a tank = 20 hrs
Formula Used
Work = Efficiency × Time
Calculation
LCM of (10, 20) = 20
Let the work be 20x
Efficiency of 1st pipe = 20x/10 = 2x/hr
Efficiency of 2nd pipe = 20x/20 = x/hr
Total time by both working together to fill the tank = 20x/(3x) = 6 hr 40 minutes
∴ The total taken by both working together to fill the tank is 6 hr 40 minutes