Science Puzzle
The Ten-Pence Blind Spot
A bottle and a coin together cost £1.10. The bottle costs exactly £1.00 more than the coin.
How much does the coin cost?
The Answer
Five pence, not ten. Almost everyone answers ten pence, because the brain reads "costs £1 more" and subtracts from £1.10. But check it: if the coin is 10p and the bottle costs £1 more, the bottle is £1.10, making the total £1.20, which is too much.
The correct algebra: coin + (coin + £1.00) = £1.10, so 2 × coin = 10p, and the coin is 5p. The bottle is then £1.05, exactly £1 more, and the total is £1.10.
The puzzle is not mathematically hard. It is a demonstration of how confident and wrong a quick mental shortcut can be, and why checking your answer against all the constraints is always worth the extra second.
The principle: Cognitive blind spots. Linguistic framing triggers a fast confident shortcut that bypasses the algebra, producing a wrong answer before the mind has checked the constraints.