Mary and Amy had a total of 20 yards of material from which to make costumes. Mary used three times more material to make her costume than Amy used, and 2 yards of material was not used. How many yards of material did Amy use for her costume?

Let x be the amount Amy used
Let 3x be the amount Mary used
Let 20-2 be the total amount used

3x + x = 20-2
4x=18
x=4.5

Amy use 4.5 yards of material.

Hope this helped!