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Two pipes can fill a tank in 10 hrs and 20 hrs, respectively. How long will they take to fill up the tank, if they are used together?
1. 6 hrs 35 min
2. 6 hrs 30 min
3. 6 hrs 40 min
4. 6 hrs 45 min

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Correct Answer - Option 3 : 6 hrs 40 min


1st pipe can fill the tank = 10 hrs 

2nd pipe can fill a tank = 20 hrs 

Formula Used 

Work = Efficiency × Time


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


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