(a) To create a proportion, we can set up the relationship between the number of rows and the number of seats. We know that 28 rows correspond to 728 seats, and we want to know how many seats (let's call it \( x \)) are in 12 rows. The proportion can be set up as follows:
\[ \frac{28 \text{ rows}}{728 \text{ seats}} = \frac{12 \text{ rows}}{x \text{ seats}} \]
(b) To solve the proportion, we can cross-multiply:
\[ 28 \cdot x = 12 \cdot 728 \]
Calculating the right side:
\[ 12 \cdot 728 = 8736 \]
So we have:
\[ 28x = 8736 \]
Now, divide both sides by 28 to solve for \( x \):
\[ x = \frac{8736}{28} = 312 \]
Thus, the answer is:
(b) \( x = 312 \)
So, in 12 rows of the stadium, there are 312 seats.