https://sites.google.com/a/wayland.k12.ma.us/6th-grade-math-resources/home/fraction-decimal-percents/equivalent-fractions
https://www.open.edu/openlearncreate/mod/oucontent/view.php?id=564&printable=1
http://fractions-math.blogspot.com/2010/12/subtracting-fractions.html