How do you hyphenate numbers?
Is it a 250-page book? Or a 250 page book?
Is it twenty-eight? Or twenty eight?
Number + Noun
Any number that precedes a noun is hyphenated.
Hence, A 250-page book is correct.
But, watch out for five- to ten-minute intervals. It has a space after the first hyphen.
Numbers, Spelled Out
Twenty-one through ninety-nine are hyphenated; numbers above one hundred are open (no hyphens). e.g. five hundred fifty
That means twenty-eight is the correct answer.