The answer is 43.
Look at the other four numbers first: 9 = 3 × 3; 16 = 4 × 4; 25 = 5 × 5; 36 = 6 × 6. They are all perfect squares — a neat staircase: 3², 4², 5², 6². The next step on that staircase should be 49 (7²)… but the list gives you 43 instead.
And 43 doubles down on being the odd one out: it isn’t just “not a square,” it’s also prime — nothing divides it but 1 and itself — while 9, 16, 25, and 36 can all be broken into smaller factors. Two different tests, same answer. That’s how you know it’s the one.
Did you spot it from the squares, or did something else give it away? Tell us your route in the comments — and watch our page for tomorrow’s puzzle.
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