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2 pipes are used to fill pool. 1st pipe can fill the pool 6 hrs alone. two pipes can fill in 3 hrs less time than 2nd pipe. how long it will take for 2nd pipe?

kindly help meeee. I'm reviewing this for my college entrance test. please help

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Mark M. | Math Tutor--High School/College levelsMath Tutor--High School/College levels
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Let x = time it takes the second pipe to fill the pool
 
Then, x-3 = time it takes both pipes together to fill the pool
 
1/6 = portion of pool filled by first pipe in one hour
 
1/x = portion of pool filled by second pipe in one hour
 
1/(x-3) = portion of pool filled by both pipes in one hour
 
So, 1/6 + 1/x = 1/(x-3)
 
Multiply by the LCD, 6x(x-3):
 
   x(x-3) + 6(x-3) = 6x
 
   x2 - 3x + 6x - 18 = 6x
 
   x2 - 3x - 18 = 0
 
   (x-6)(x+3) = 0
 
   x = 6 or x = -3
 
Since x can't be negative, x = 6 hours