To calculate Manny's earnings for the given hours worked, we can substitute the values into the expression \(21a + 12.50d\), where \(a\) is the number of hours worked as an aerobics instructor and \(d\) is the number of hours worked at the donut shop.
Given:
- \(a = 4\) (hours as an aerobics instructor)
- \(d = 8\) (hours at the donut shop)
Now, substitute the values into the expression:
\[ Earnings = 21(4) + 12.50(8) \]
Calculating each term:
\[ 21(4) = 84 \] \[ 12.50(8) = 100 \]
Now, add the two amounts:
\[ Earnings = 84 + 100 = 184 \]
Therefore, Manny will earn $184 working 4 hours as an aerobics instructor and 8 hours at the donut shop.
The correct response is: $184