Should you put a number or spell out the word when preceding a unit of measurement?
APA manual, page 111 says to use number but Purdue OWL doesn't state one way or the other.
Answer
The Publication Manual of the American Psychological Association, 6th edition, is the Ur source. It takes precedence over any other supplemental style guide, such as the Owl of Purdue.
You are correct, according to the APA manual you should use the number (i.e. numerical form) when it precedes a unit of measurement.
The only thing that supersedes the APA manual is "local custom," meaning you professor can tell you to type your paper in white 48 point font and spell out all numbers backwards and, well, that is what you do. ;)