You might do this if you discover an error or ambiguity in a problem:

1. Under the Instructor Tools menu, go to “Hmwk Sets Editor”
2. Click on the number (will look like 15/15 etc.) under the column “Edit Problems” for that homework
3. Then click on the “Mark Correct” box for the specified problem
4. Don't forget to click “Save Changes” at the bottom of the page!