To find out how much Manny will earn working 4 hours as an aerobics instructor and 8 hours at the donut shop, we can substitute \(a = 4\) and \(d = 8\) into the expression \(21a + 12.50d\).
So, we calculate:
\[ 21(4) + 12.50(8) \]
Calculating each part:
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For the aerobics instructor: \[ 21 \times 4 = 84 \]
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For the donut shop: \[ 12.50 \times 8 = 100 \]
Now, we sum the two amounts:
\[ 84 + 100 = 184 \]
Thus, Manny will earn $184 working 4 hours as an aerobics instructor and 8 hours at the donut shop.