Answer:
We can use the given formula to solve for the cost C:
P = R - C
We know that P = $49.32 and R = $81.18, so we can substitute these values into the formula:
$49.32 = $81.18 - C
Next, we can solve for C by isolating it on one side of the equation:
C = $81.18 - $49.32
C = $31.86
Therefore, it cost Zach $31.86 to make the jewelry he sold at the fair.