Hyphen Usage—Rules and Examples - Grammarly
https://www.grammarly.com/blog/hyphen14.01.2021 · Hyphen in Compound Adjective With Numbers. When numbers are used as the first part of a compound adjective, use a hyphen to connect them to the noun that follows them. This way, the reader knows that both words function like a unit to modify another noun. This applies whether the number is written in words or in digits.