#### Problem

*If the numbers 1 to 5 are written out in words: one, two, three, four, five, then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.*

*If all the numbers from 1 to 1000 (one thousand) inclusive were written out in words, how many letters would be used?*

**NOTE:** Do not count spaces or hyphens. For example, 342 (three hundred and forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20 letters. The use of “and” when writing out numbers is in compliance with British usage.

#### Solution

let onesToWord prefix n postfix =
match n with
| 1 -> prefix + "one" + postfix
| 2 -> prefix + "two" + postfix
| 3 -> prefix + "three" + postfix
| 4 -> prefix + "four" + postfix
| 5 -> prefix + "five" + postfix
| 6 -> prefix + "six" + postfix
| 7 -> prefix + "seven" + postfix
| 8 -> prefix + "eight" + postfix
| 9 -> prefix + "nine" + postfix
| _ -> ""
let tensToWord prefix tens ones =
match tens with
| 0 -> onesToWord prefix ones ""
| 1 -> match ones with
| 0 -> prefix + "ten"
| 1 -> prefix + "eleven"
| 2 -> prefix + "twelve"
| 3 -> prefix + "thirteen"
| 4 -> prefix + "fourteen"
| 5 -> prefix + "fifteen"
| 6 -> prefix + "sixteen"
| 7 -> prefix + "seventeen"
| 8 -> prefix + "eighteen"
| 9 -> prefix + "nineteen"
| _ -> ""
| 2 -> prefix + "twenty" + (onesToWord "" ones "")
| 3 -> prefix + "thirty" + (onesToWord "" ones "")
| 4 -> prefix + "forty" + (onesToWord "" ones "")
| 5 -> prefix + "fifty" + (onesToWord "" ones "")
| 6 -> prefix + "sixty" + (onesToWord "" ones "")
| 7 -> prefix + "seventy" + (onesToWord "" ones "")
| 8 -> prefix + "eighty" + (onesToWord "" ones "")
| 9 -> prefix + "ninety" + (onesToWord "" ones "")
| _ -> ""
let toWord n =
let thousands = n / 1000
let hundreds = (n - 1000 * thousands) / 100
let tens = (n - 1000 * thousands - 100 * hundreds) / 10
let ones = n % 10
let thousandsWord = onesToWord "" thousands "thousand"
let hundredsWord = onesToWord "" hundreds "hundred"
let tensPrefix = if (thousands > 0 || hundreds > 0) && (tens > 0 || ones > 0)
then "and"
else ""
let tensWord = tensToWord tensPrefix tens ones
thousandsWord + hundredsWord + tensWord
let answer = [1 .. 1000] |> List.map toWord |> List.sumBy (fun s -> s.Length)