Ms. lee cut a piece of yarn into parts.
1/12, 1/4, and 5/12 what fraction of yarn is left?

If you give all the fractions common denominators, they will be 1/12, 3/12, and 5/12.  Add the fractions together to get 9/12.  Subtract that from 1 or 12/12.  This will give you the result of 3/12 or 1/4 therefor making the answer 3/12 or, even better, 1/4 of the yarn being left.

Then just do it (lol)