From a book I just finished:
"[The company] wants each new employee evaluated at thirty, sixty, and ninety-day intervals..."
When you have a compound adjective such as "ninety-day," you need a hyphen, as the author so nicely included. You also need a hyphen with the other numbers since they all modify "intervals": thirty-, sixty-, and ninety-day intervals."
The beginning two hyphens seem to just be hanging there, but they're waiting to be partnered up with "day" later in the sentence. You could repeat "day" three times, but that wouldn't sound so good.
Ask Me a Question
If you have a writing, grammar, style or punctuation question, send an e-mail message to curiouscase at sign hotmail dot com.
Add Your Own Criminal Sentence!
If you find a particularly terrible sentence somewhere, post it for all to see (go here and put it in the Comments section).