Answer
Lower unit cost is $0.099/oz, so the best buy is the 18 oz jar of peanut butter.
Work Step by Step
$\frac{1.29}{12 oz}$ = 0.108/oz
$\frac{1.79}{18 oz}$ = 0.099/oz
$\frac{3.39}{28 oz}$ = 0.121/oz
$\frac{4.39}{40 oz}$ = 0.110/oz
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