pieces of yarn .... 1/8 yard long (each)
a piece of yarn ... n = 3 yards long
remaining ... 1 1/4 yards = 5/4 yards
If you would like to calculate how many 1/8 yard pieces can Sarah cut and still have 1 1/4 yards left, you can do this using the following steps:
n - 1 1/4 = 3 - 1 1/4 = 3 - 5/4 = 12/4 - 5/4 = 7/4 = 1 3/4 yards
7/4 / 1/8 = 7/4 * 8/1 = 14 pieces
Result: Sarah can cut 14 1/8 yard pieces and still have 1 1/4 yards left.