Hyphenate all compound numbers from twenty-one through ninety-nine. Again, if timings are vague it is fine to write them out in words. In fact, if you then read point 3 you will see that there is no standard rule for those numbers, some authors like to write them in digits, others still prefer to spell them out.
When you reach numbers in the millions and billions, write out the full word instead of all those zeros.
Incorrect: The circulation of library materials increased by four point five percent in He counted out fifty-pence pieces. If the number is less precise, it may be possible to write the number in words.
First things first, what is the difference between a number and a numeral?