Use the Combine tool to create a new set out of existing sets, or to study several sets at once.
How to combine sets
Click on the set title to get to the set's main page. Hover over "More" in the Tools section on the upper right, then click "Combine."
On the next page, click on the plus button next to each set you would like to combine into one. You can add sets that you've created and sets in your classes and folders.
Then select "Create a set" from the drop-down under the "Currently Combining" section on the left, and click "Go."
You can also select a study mode if you'd like to just study your combination in one of the modes. Select the mode from the drop-down options, and click "Go"
Deleting a combination
If you created a new combined set, you can delete it by going to the main set page and clicking on "Delete" under "More."
If you were just studying a combination in a mode, you can remove this activity from your Latest Activity feed. On the left sidebar on the main logged in page, click on "Your Sets & Classes," then go to your "Studied" tab. Click on "Hide" to remove this combination.