x lbs * 5.5 + 100 lbs * 6.25 = (x+100) * 6.00
distribute
5.5x + 625 = 6x+600
subtract 5.5 x from each side
625 = .5x +600
subtract 600 from each side
25 = .5x
divide each side by .5
x = 100
You need 100 pounds of the 5.50 mix
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