To determine the constant of proportionality for Sarah's earnings, you can use the relationship that the amount earned (\(E\)) is directly proportional to the number of hours worked (\(H\)). This can be expressed with the formula:
\[ E = k \cdot H \]
where \(k\) is the constant of proportionality (the rate of pay per hour).
From the information given:
- Sarah earns \(E = 320\) dollars
- She works \(H = 40\) hours
Now, we can substitute these values into the formula and solve for \(k\):
\[ 320 = k \cdot 40 \]
To find \(k\), divide both sides by 40:
\[ k = \frac{320}{40} \]
Calculating this gives:
\[ k = 8 \]
So the constant of proportionality, which represents Sarah's earnings per hour, is:
\[ k = 8.00 \]
Thus, the answer is:
$8.00 per hour.