Tom is rational.

Tom isn't a rational guy.

Man is not a rational animal; he is a rationalizing animal.

Vietnam's public debt to GDP ratio is nearing the government-mandated ceiling of 65%.

The screen has an aspect ratio of 4:3.

He tried to rationalize his foolish actions.

This idea is not rational.

Of all the ways of defining man, the worst is the one which makes him out to be a rational animal.

The government rationed meat during the war.

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