Question 337083
P = pineapple juice @ $5.50/gal
O = orange juice @ $3.00/gal 
T = target @ $3.50/gal 

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the ratio of P & O needed will be in inverse proportion to distance from T
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P:O = (3.50 - 3.00) : (5.50 - 3.50) = 0.50:2 = 25:100

ans:
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25 gal of pineapple juice
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the conventional method is always there !
suppose x gal of pineapple juice, then
5.5x + 3*100 = 3.5*125
5.5x = 437.5 - 300 = 137.5
x = 137.5 /5.5 
= 25 gal
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but surely more complex !