How do variables you model real-world situations?

Using variables, we can represent many real life situations where the price, quantity or value of an object may be unknown. For example, many payments come in the form of monthly payments and a starting cost. Take a gym membership for example. Let y be the amount of money you have paid to the gym, x be the number of months you have gone.
y = 30x + 65
This equation models the amount of money you've paid to the gym given that the monthly fees is $30 and the registration fees was $65