Each of these numbers is written in plain block digits on a card held above still water. For which number does the water image (the reflection flipped top to bottom) look exactly the same as the number itself?

A water image flips each digit top to bottom but keeps the digits in the same left-to-right order. Only the digits 0, 1, 3 and 8 are unchanged by a top-to-bottom flip. In 1881 every digit is either 1 or 8, so the reflection reads 1881 again. The other numbers contain 9, 6, 2 or 5, which change shape when flipped.