It's very hard to understand concepts like prevalence, incidence, and case fatality ratio, but some articles explain these very well.

The case is not a rational case.

The screen has an aspect ratio of 4:3.

No rational person, of whatever political persuasion, could possibly object to the program.

Tottie isn't always rational.

The sum of two rational numbers is also rational.

We're rational.

But if we were completely rational, would we then still want children?

Muri arranged the letters on the front of her letterbox in the form of the golden ratio.

Supplies of food were low and we had to ration out the little that was left.