Every Saturday, Workout Nation holds a 45 minute aerobics class. Weekday aerobics classes last 30 minutes. The number of weekday classes varies. Last week there were a total of 165 minutes of aerobics classes available. How many weekday aerobics classes were there.

Okay. A function for this situation would be m = 300 - 20x. This is because Carla has $300 left in the bank account and she take out $20 per withdrawal.  As she withdraws more money from the bank, the total amount of money in there gets less and less.