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There are many examples of linear relationships that you could use.
On the one hand, you could create a situation that is perfectly linear, such as determining the total income of the local paper boy as a function of the number of papers that he delivers. The income function will have the form y = mx + b, where y is the paper boy's income, m is the income per paper, x is the number of papers, and b is any fixed income (which could also be zero).
I suspect, however, that the instructions that were given to you suggest that you find a data set that is linear-ish, rather than coming up with a perfectly linear equation. Finding suitable real data can be difficult (and exhausting). A popular approach is to select an Olympic sport (because records are easy to obtain, see www.olympic.org), and record the times in that event over several years. Surprisingly, over the long-term, the improvements in times tends to be close to linear.
If it is permissible to create a fictitious data set, you could also "record" the height and weight of classmates. Provided that you avoid those who deviate extremely from "normal", the relationship should be fairly linear. You could also make up something about the number of fruit trees that various farmers have and the total yield of their crop, or the number of books in a student's backpack and the total weight of the pack. Pretty much anything that is more or less proportional will work.
I assume that you are referring to the Olympics suggestion. Keep in mind that there are many events in both the Summer and Winter games, as well as Men's vs Women's events. There are many to pick from. My interpretation of the instructions is that you need to have a data set that is different from the other students, meaning that two students shouldn't be using the exact same Olympic event.
Of course, if your instructor has already steered you away from that topic, then the decision has been made for you.
Visit the Olympics page at www.olympic.org, and click on the "Sports" tab near the top of the page.
On the Sports page, down a little ways on the right hand side is a box titled "Games Results". Use the settings in that box to select a specific Olympic year, a sport, and a specific event. Then click on "View Event Results".
When you get the results page, record the time for the gold medal winner in that event.
You can then use the drop-down menus at the top of the page to get results for that same event in a different year. All you need to do is change the first of the three drop-down menus.
Record the times for at least 8 Olympic years.
Once you have the data that you want to use, you can send it to your instructor for approval, if necessary.
When you are ready to move on with the assignment, just send me the data and I can do the rest.