Problems should use the Courier font. Why? Every Courier font character uses the same amount of space. A comma is the same width as the number 5. This means that all of the numbers line up perfectly for carrying during addition and bringing the zero down in division.
Books _ Among the dozen or so math word_problem books of note published in the past decade, one, written by Mary Jane Sterling, from a well_known, popular brand of information books, which I'm unable to name here, stands out. The publisher has put a decent chunk of the book online at books.Google.com (go to books.google.com and search for sterling word problems).