Required attendance for condo association board quorum in Illinois

What constitutes a condo association board quorum for executive session?  Our condo board is 7 members. However, 2 have recently resigned. At this point, If the board holds an executive session meeting and need quorum (50%?), would it be 3 members (50% of 5) or 4 members (50% of 7)?

I could not find this answer in either the Condo Property Act or our Bylaws. Also, I believe our Bylaws are quite outdated. Thank you.