Question 906027
there at two pipes to the bathtub.  You turn the first pipe on and the bathtub fills in 5 mins.  You empty the bathtub.  The you turn pipe number 2 on and it takes 7 mins to fill up.  Then you empty the bathtub and turn both pipes on.  How long does it take to fill up the bathtub?
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The 1st pipe fills 1/5 of the tub per minute
The 2nd pipe fills 1/7 of the tub per minute
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Together, they fill 1/5 + 1/7 = 12/35 per minute
--> 35/12 minutes/tub