A better response would be honesty: "I've seen this question before on a (puzzle/interview prep/whatever) site. The optimal answer was better than what I initia

A better response would be honesty: "I've seen this question before on a (puzzle/interview prep/whatever) site. The optimal answer was better than what I initially came up with... I didn't consider that we knew one of the coins was explicitly heavier than the others, I was only thinking that one was a different weight. That's a good reminder to read your specs very closely." Or something like that.