That’s not the question. Either the scales balance, and the third is heavier or lighter, or the scales don’t balance and you get both answers, but the question is purposely framed this way
That’s easy enough to answer, but he really should work on his grammar. In that case you just do 3 groups of three, weigh two of them. If they’re even, the third group is different. Weigh 2 membres of the third group, they’ll either be even or one heavier. Weight the last member against the heavier one from step 2 to see if they’re even or not for your answer.
Programmer tries to explain binary search to the police (startrek.website)