Re: How do you use ratios and proportions in real life?

Well, the easiest example I can think of to use would be in cooking. Scaling a recipe up or down depending on the number of guests you have would be a wonderful example.

But, I assume you need something a little more scientific. Ok, how about the following:

In order to determine how long a someone can work in an area with a lot of radiation, first we take a measurement of the radiation field. In general, we have an idea how long specific jobs will take. We can use ratios and proportions to scale up or down the time they are allowed to work in a particular area. Health physics uses a lot of proportions to give an "on the fly" estimation of radiation dosage.

Another example would be in the field of medicine. Ratios and proportions are used to determine proper medication dosage for a patient if you have to change it for body mass, age, etc.