$2.40/kg -----> 1/(1+3) * 40 = 1/4 * 40 = 10 kg
$3.20/kg -----> 3/(1+3) * 40 = 3/4 * 40 = 30 kg
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$2.40/kg * 10kg + $3.20/kg * 30 kg = $24.00 + $96.00 = $120.00
$120.00 / 40 kg = $3.00/kg
https://answers.yahoo.com/question/index?qid=20111211002452AA7m4Tv
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