SOLUTION: An inlet pipe can fill Dominica's pool in 7 hr, while an outlet pipe can empty it in 11 hours. In his haste to watch television, Dominica left both pipe open. How long did it take

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Question 184242: An inlet pipe can fill Dominica's pool in 7 hr, while an outlet pipe can empty it in 11 hours. In his haste to watch television, Dominica left both pipe open. How long did it take to fill the pool.
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
An inlet pipe can fill Dominica's pool in 7 hr, while an outlet pipe can empty
it in 11 hours. In his haste to watch television, Dominica left both pipe open.
How long did it take to fill the pool.
:
TV is a brain destroyer, avoid it at all costs!
:
Let t = time required when both pipes are open
:
Let the completed job = 1 (full pool)
:
Fill is +, emptying is -
:
t%2F7 - t%2F11 = 1
Multiply equation by 77:
77*t%2F7 - 77*t%2F11 = 77(1)
cancel out the denominators:
11t - 7t = 77
4t = 77
t = 77%2F4
t = 19.25 hrs or 19 hrs 15 min
:
:
Check solution
19.25%2F7 - 19.25%2F11 = 1
2.75 - 1.75 = 1, confirms our solution