To find the amount \( a \) Mike paid for the 8 yards of ribbon, we can set up the equation based on the cost per yard.
First, we know that the cost per yard is $3.50. Therefore, the total amount \( a \) that Mike paid for 8 yards can be calculated as:
\[ a = \text{cost per yard} \times \text{number of yards} \]
Thus, the equation representing this relationship is:
\[ a = 3.50 \times 8 \]
Now, let's complete the working equation.
Starting from the provided equation format:
- Rewrite it according to the original form given: \[ \frac{a}{8} = 3.50 \]
Now, we can find the value of \( a \):
\[ a = 3.50 \times 8 = 28.00 \]
So the completed working equations are:
\[ \frac{a}{8} = 3.50 \]
\[ a = 28.00 \]
Therefore, Mike paid \( \text{$}28.00 \) for 8 yards of red ribbon.