Avoid o as a variable, make numbers mathy and restructure

Summary: Ensure no one's name starts with 'O' in case it is confused with 0. I'm not sure how likely that is. I also made the numbers mathy and removed some needless divs and opening and closing code tags. And restructed things a bit becuase I couldn't help myself.

Test Plan: Make sure /exercises/age_word_problems.html?seed=169&problem=solve-older-1&debug&lang=en doesn't involve Omar. Make sure the hints for all the exercise types still make sense.

Reviewers: eater, emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D13905