Handful of tweaks to some new exercises

Summary:
- Also remove the period from "None of the above" since it's not a sentence.
- Fix a bug with the multipler when multiplying decimals where you don't have to add (i.e., the second multiplicand has only one digit not countnig leading zeros).

Test Plan: View each affected exercise in local mode, clicking through all hints; Also run unit tests

Reviewers: joel

Reviewed By: joel

CC: alpert

Maniphest Tasks: T2012

Differential Revision: http://phabricator.khanacademy.org/D5627