Erica bougfht 3 1/2 yards of fabric if she uses 2/3 of the fabric for a curtain. how much will she have left?

If she uses 2/3 of the fabric, she has 1/3 of the fabric left.

1/3 of 3 1/2 yd = 1/3 * 3 1/2 yd = 1/3 * 7/2 yd = 7/6 yd = 1 1/6 yd

She has 1 1/6 yd of fabric left.

Extra explanation:
You can do it two ways. I showed it to you one way. What I did above was: a whole thing is 3/3 of the thing since 1 = 1/1 = 3/3.
The entire amount she bought is one unit. It ios the same as 3/3 of what she bought. Since she used 2/3 of what she bought, she had 1/3 of what she bouht left. Then I found out what 1/3 of 3 1/2 is.

Here is a different way. She used 2/3 of what she bought, so find first what 2/3 of 3 1/2 yards is. Then subtract it from 3 1/2 yd.

2/3 * 3 1/2 = 2/3 * 7/2 = 14/6 = 7/3

3 1/2 - 7/3 = 7/2 - 7/3 = 21/6 - 14/6 = 7/6 = 1 1/6

As you can see, you still get the same answer, 1 1/6 yd left.