Background:
Open middle tasks allow students to use a range of strategies to attempt to find a solution but do not have an open end, i.e. there is only one correct solution. The benefit of using these problems is the rich discussion students can have around the various strategies used (open middle) to find the solution (closed end)
Resource:
A huge collection of Open Middle problems can be found at the Open Middle website: https://www.openmiddle.com/