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?
MY SET UP:
Let M = Mary
Let A = Amy
M + A = 20 yards
3M + A = 18 yards
I equated 3m + A to 18 because the question stated that 2 yards were not used.
Is this the correct group of equations or is the entire set up wrong?