Question 378673
If I have four gallons of 50 degree farenheit water and I need to add one more gallon of water what does the temperature of the last gallon need to be if the whole mixture ends up at 70 degrees.
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If you assume that mixing equal volumes of water gives the mix the average of the 2 temperatures, which seems likely, tho I haven't thought about this before:

Then you have to deal with units of "degree-gallons", something new to me.
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4 gals @ 50º = 200 deg-gallons
The mix desired is 5 gallons @ 70º = 350 deg-gallons
The gallon to be added has to have 150 deg-gallons of heat, so it has to be 150º