SOLUTION: Pipe A can fill a tank in 4 h. Pipe B can fill the tank in 9 h less than the time it takes pipe C, a drain pipe, to empty the tank. When all 3 pipes are open, it takes 2 h to fil

Algebra ->  Test -> SOLUTION: Pipe A can fill a tank in 4 h. Pipe B can fill the tank in 9 h less than the time it takes pipe C, a drain pipe, to empty the tank. When all 3 pipes are open, it takes 2 h to fil      Log On


   



Question 607135: Pipe A can fill a tank in 4 h. Pipe B can fill the tank in 9 h less than the time it takes pipe C, a drain pipe, to empty the tank. When all 3 pipes are open, it takes 2 h to fill the tank. How much time is required for pipe C to empty the tank if pipe A and B are closed.
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Add the rates of filling and subtract the
rate of emptying
A's rate: ( 1 tank filled ) / ( 4 hrs )
B's rate: ( 1 tank filled ) / ( C's rate minus 9 hrs )
C's rate: ( 1 tank emptied ) / ( x hrs )
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When all 3 pipes are open, it takes 2 hrs to fill the tank.
+1%2F4+%2B+1%2F%28+x-9+%29+-+1%2Fx+=+1%2F2+
Multiply both sides by +4%2Ax%2A%28+x-9+%29+
+x%2A%28+x-9+%29+%2B+4c+-+4%2A%28+x-9+%29+=+2%2Ax%2A%28+x-9+%29+
+x%5E2+-+9x+%2B+4x+-+4x+%2B+36+=+2x%5E2+-+18x+
+x%5E2+-+9x+=+2x%5E2+-+18x+-+36+
+x%5E2+-+9x+-+36+=+0+
Use quadratic formula
+x+=+%28-b+%2B-+sqrt%28+b%5E2+-+4%2Aa%2Ac+%29%29+%2F+%282%2Aa%29+
+a+=+1+
+b+=+-9+
+c+=+-36+
+x+=+%28-%28-9%29+%2B-+sqrt%28+%28-9%29%5E2+-+4%2A1%2A%28-36%29+%29%29+%2F+%282%2A1%29+
+x+=+%28+9+%2B-+sqrt%28+81+%2B+144+%29%29+%2F+2+
+x+=+%28+9+%2B-+sqrt%28+225+%29%29+%2F+2+
+x+=+%28+9+%2B+15%29+%2F+2+
+x+=+24%2F2+
+x+=+12+ ( ignore the negative square root )
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If A and B are closed, C can empty the tank in 12 hrs