Which Number Is Spelled in Alphabetical Order? Riddle Answer

The answer is 40. Spelled out in English, FORTY uses the letters F, O, R, T and Y, and each one comes later in the alphabet than the letter before it.

The riddle: which counting number, spelled out in English, has all of its letters in alphabetical order?

How can you check that forty works?

Write each letter's place in the alphabet next to it.

The places climb every time: 6, 15, 18, 20, 25. That is what perfect alphabetical order means here, with no letter repeated and none stepping backwards.

Is forty the only answer?

Yes, for counting numbers written the usual way in English. The order word FIRST also has its letters in order, but it names a position, not a number. Every number word from zero to ninety-nine breaks the order except forty. A few examples show how quickly the others fail:

• ONE goes from O to N, and N comes before O.
• SIX goes from S to I, which steps back at once.
• FIFTY goes from I back to F.

Bigger numbers fail too. HUNDRED steps back from U to N, THOUSAND steps back from T to H, and MILLION steps back from M to I, so every number that needs one of those words is out.

Why is it easy to miss?

If you start with the small numbers, it is easy to give up after the first twenty all fail. Spelling it FOURTY, with a U, also breaks the order. Checking every case is how many classic puzzles are settled. By 2018 a computer search had shown that no packing of squares of side 1 to 24 into a 70 by 70 square beats one that leaves out only the 7 by 7 square ^[1], and the sliding 15-puzzle, was known by 2015 to be solvable in 80 moves or fewer ^[2].

What is a quick way to test number words?

Read the word and ask one question at each letter: does this letter come after the last one? The first time the answer is no, move on. You can rule out most number words within two or three letters.

Want another word riddle?

The related riddles below play with letters in other ways.