MHB If both pipes are used together, how long will it take to fill 2/3 of the tank?

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One inlet pipe fills an empty tank in 5 hours. A second inlet pipe fills the same tank in 3 hours. If both pipes are used together, how long will it take to fill 2/3 of the tank?

My Work:

Let x = time when both pipes are used together

(1/5) + (1/3) = 1/x

I found x to be 15/8 hours.

Must I now multiply (15/8)(2/3)?
 
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RTCNTC said:
I found x to be 15/8 hours.

Must I now multiply (15/8)(2/3)?
Correct.
Yes.
 
Wilmer said:
Correct.
Yes.

It took several tries before I found the correct set up. Unfortunately, no such thing as ENOUGH TIME when taking a test.
 
That's why you "practice, practice, practice" before the test!
 
Country Boy said:
That's why you "practice, practice, practice" before the test!

My classroom days ended in December 1993.
 
Then what "test" were you talking about?
 
Test tickle?
 
As a slightly different way to approach this problem, we can see that working together for 15 hours, the two inlet pipes can fill 8 tanks, and so it would take 15/8 hours for the two pipes to fill one tank, and 2/3 of that time to fill 2/3 tank, since the two pipes flow presumably at constant rates. Hence:

$$t=\frac{2}{3}\cdot\frac{15}{8}\text{ hr}=\frac{5}{4}\text{ hr}$$
 
Wilmer said:
Correct.
Yes.

(15/8)(2/3) = 5/4 hrs
 
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Stick a pink star on your forehead :)
 
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