(a) To find the number of t-shirts Susan sold, we can use the following equation:
Profit = Revenue - Expenses
Here, her profit is $84, her revenue from selling t-shirts is \( 6x \) (where \( x \) is the number of t-shirts sold), and her expenses are $30. Therefore, we can set up the equation as follows:
\[ 84 = 6x - 30 \]
(b) Now, we'll solve the equation \( 84 = 6x - 30 \).
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Add 30 to both sides:
\[ 84 + 30 = 6x \] \[ 114 = 6x \]
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Divide both sides by 6:
\[ x = \frac{114}{6} \] \[ x = 19 \]
So, Susan sold 19 t-shirts.