adjective
1
BAD : I was far too upset and emotional to make a reasonable decision.
GOOD : I was far too upset and emotional to make a rational decision.
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reasonable = fair, sensible or acceptable: 'Dividing up the work equally seems like a very reasonable decision.' 'They'll accept any reasonable offer.'
rational = produced by means of careful, logical thinking: 'There must be some rational explanation - things can't just disappear!'
2
BAD : She usually gets reasonable good marks.
GOOD : She usually gets reasonably good marks.
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When you mean 'fairly/quite', use reasonably (adverb): 'The team played reasonably well on Saturday, but something was missing in the attack.'