Multiplying fractions by integers, trello #555

Works now, thanks to @davemajor and @petercollingridge 

Trello card:
https://trello.com/card/multiplying-fractions-by-a-whole-number/4d87e664967a0775082939ab/555

Sandcastle:
http://sandcastle.khanacademy.org/media/castles/ifingerson:fraction_x_whole_num/exercises/multiplying_fractions_by_integers.html