Which number is the odd one out: 15, 47, 24, 35?

Add 1 to each and the tie shows itself: 15 + 1 = 16, 24 + 1 = 25 and 35 + 1 = 36, every one a perfect square. 47 + 1 = 48, which is no square — 49 would have been. The near miss is the trap, since 47 sits only two below a square and reads as orderly as the rest.