Khan/khan-exercises — pull request #10820 by dsissitka (2012-01-10 05:06:15+00:00)
Title: Combined "Adding fractions with common denominators" and "Subtracting fr...

...actions with common denominators" into "Adding and subtracting fractions with like denominators".

This is re [Understanding decimals](https://trello.com/card/board/understanding-decimals/4d87e664967a0775082939ab/4eeed67b90148cdc1f10fa26).