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**