Question 286490
Pipe A can fill a tank in 30 minutes. Pipe B can feel the same tank in 20 minutes. How long will it take to fill the tank using both pipes at the same time? 
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A rate: 1/30 tank/min
B rate: 1/20 tank/min
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Together rate: 1/x tank/min
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Equation:
rate + rate = together rate
1/30 + 1/20 = 1/x
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Multiply thru by 60x and solve for "x":
2x + 3x = 60
5x = 60 
x = 12 minutes (time to fill the tank using both tanks)
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Cheers,
Stan H.