rational — as in: rational behavior
It’s hard to think rationally when I’m this upset.

reasonable — in various senses: logical as contrasted to illogical
 based on reason as contrasted to emotion
 capable of using reason as contrasted to being insane or lacking the ability to reason
 based on a logical analysis as contrasted to intuition, instinct, custom, tarotcard reading, or some other system of decision making
More rarely, rational numbers in mathematics are numbers that can be expressed as a fraction consisting of only an integer in the numerator and denominator. For example, 1/3 is a rational number while pi is an irrational number.
