The correct interpretation of the function \( y = 15x + 12 \) in the context of the gym's pricing model is that:
\( y \) represents the total cost, and \( x \) represents the number of months of membership. The equation indicates that there is a one-time enrollment fee of $12 and a monthly fee of $15.
Therefore, the correct statement is:
y is the cost and x is the number of months someone is a member of the gym; $15 is the monthly fee and $12 is the enrollment fee.