Add exercise: converting improper fractions and mixed numbers for simple...

... conversions

@mwahl, any feedback?
I revised the hints on this exercise, removing long division (this is covered by other Khan exercises). I welcome any feedback.http://sandcastle.khanacademy.org/media/castles/clkirby1:cfmd/exercises/converting_fractions_and_mixed_numbers_to_decimals_1.html .This exercise was proposed in the Google Doc (Fractions and Decimal Exercises), and on Trello, https://trello.com/card/fraction-decimal-conversions/4d87e664967a0775082939ab/320 . There are links in the hints to Khan videos. The links don't seem to function on Sandcastle
I am unclear from the exercise description if this is to be in addition to the existing exercises or as a replacement.