We teach students ways to determine if two ratios are proportional but do we ever really spend time showing students its multiple uses in real life?
Out in the bush, the first application of proportions is in making clothing. They base it on a person's own body unit such as inch for the finger, or yard via the arms. They say they create the pattern this way so its right for the wearer.
Another situation is when you have to enlarge or decrease a recipe. You use proportions to determine the new amount of each ingredient so your yield is the right amount. Often, I find recipes created to make food for a family of 6 and there is only me, so I reduce it by about 2/3 so I've got enough for me for a couple of meals.
At the end of a snowstorm, its easy to calculate the amount that fell on average each hour, during the storm. Again, its the total amount/total time = amount/hour which is often done by the weather channel. Its much easier to do an average rather than trying to calculate it hour by hour. I don't know if you've been out in a blinding snow storm but I have and I never stop to figure out how fast its falling.
This particular example comes up in a variety of math but its true that currency exchange uses proportions. You have the exchange rate of say $1.00 American = $.70 Canadian so how much would you get if you wanted to change $35.00 American into Canadian? I know about those exchange rates because my sister used to work for a customs house and I had to help her figure out how much to bill a customer in Canada.
Proportions play a large part in art when an artist uses proportion to get the look right on items in a picture. In Ancient Egypt, they proportioned the body so it was in thirds, even the children so the body from waist up was 1/3rd, the waist to the knees, and the knees down.
Even architects use proportions to create buildings that "look right" If the proportions are wrong, the building looks wrong. If you check out this place, it is a download which looks at the different types of proportion and ratios used in Architecture. It comes with worksheets and explains the application of the golden ratio as used by architects.
Its nice to have items to discuss in further detail when noting real life applications of proportion other than the standard ones from the textbook. If you find more examples, let me know as I"m always open for suggestions.