Explain a real-world problem that you used math to solve. What mathematical expressions or equations did you use in your problem solving? Define your variables and explain your expression. After you've provided one worked out example, include a similar problem for your classmates to work.
Another example might be if you need to create some kind of enclosure. Lets say you have a limited amount of fencing available and you want to make a rectangular enclosure. What should the dimensions of your rectangle be in order to maximize the area encompassed by the enclosure? I'm not sure what level of math you are talking about, but this is a real-world example of where you might apply calculus.
Some other ideas include balancing a budget (arithmetic), cooking from a recipe (adding, and sometimes converting fractions), converting currencies (Pre-algebra), optimizing DPS while playing World of Warcraft (Algebra I), and understanding compounding credit rates (precalculus). I'm sure you can think of plenty more.