21. A fitness club charges \( \$ 25 \) per month as well as a one-time \( \$ 80 \) fee of al new members. Which equation represents the cost in dollars, \( c \), for a nen member who prepays for \( m \) months? A \( c=105 m \) B. \( c=m+105 \) C. \( c=80 m+25 \) D. \( c=25 m+80 \)

To find the total cost for a new member who prepays for \( m \) months at a fitness club, we need to add the one-time fee and the monthly charges together. The total monthly cost for \( m \) months is \( 25m \), and the one-time fee is \( 80 \). So, the correct equation that represents the cost \( c \) is \( c = 25m + 80 \), which corresponds to option D. In this scenario, it's important to break down costs into fixed and variable components. The one-time fee ($80) is fixed irrespective of how many months you pay for, while the monthly fee ($25) varies based on your prepaid months. Always ensure you identify these components to avoid miscalculating costs!