use number or write it out? one sentence.

In America, a family of six is in poverty when the annual income is below $29,000.

Use the number, it looks much cleaner and neater

The rules of style change depending on which set of style guidelines you use -- MLA, Chicago, APA, etc...
Recently the American Psychological Association released the 10th edition of the APA style guide book, and one of the changes was this: All numbers 10 and below should be written as words (like Varchas suggests above), and numbers above should be written as numerals.

APA used to recommend that numbers twenty and below should be written as words, but now they changed it to ten.

:-)