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Math Problem.

Posted: Tue Apr 11, 2006 9:09 pm
by s.dot
Someone posted this on my forums, and I solved it.

Then she found a link to the math problem and it was a different answer

The Problem:
Pipe A fills a tank in 30 minutes. Pipe B can fill the same tank 5 times as fast as pipe A. If both the pipes were kept open when the tank is empty, how much time will it take for the tank to overflow?
MY Logic:

Pipe A fills the tank in 30 minutes.
Pipe B = 5 times faster, so it will fill it in 6 minutes.

If you put them together, pipe A is helping out by 1/5th.

6 minutes in seconds is 360 seconds.
1/5th of 360 is 72.

360 seconds - 72 seconds = 288 seconds.

288 seconds = 4.8 minutes.

And 4.8 minutes = 4 minutes and 48 seconds

Thus, my answer = 4 minutes and 48 seconds

This page does it differently.

Their answer: 5 minutes

If I'm wrong, how so?

Posted: Tue Apr 11, 2006 9:33 pm
by feyd
1x + 5x = 6x
30 / 6 = 5

Posted: Tue Apr 11, 2006 9:35 pm
by s.dot
So where did I go wrong in my logic?

Posted: Tue Apr 11, 2006 10:02 pm
by RobertGonzalez
You were subtracting in your logic. 'A' helping 'B' is additive...

Pipe{a} = 1 Tank / 30 Minutes
Pipe{b} = Pipe{a} * 5
Pipe{b} = 1/30 * 5
Pipe{b} = 1 Tank / 6 Minutes

Pipe{a} + Pipe{b} = 1/30 + 1/6
=> 1/30 + 5/30
=> 6/30
=> 1 Tank / 5 Minutes