A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I'm with Phillip on this one, except that LibreOffice has the ability to save in current Excel format (.xlsx), so choose that format.
LibreOffice may change all your Office file associations without your permission each time you open it. Once you have finished using LibreOffice, drag it to the trash and empty the trash.
For each kind of Office document, in Finder right click on a saved file. Choose Get Info. Change Open With and choose the Microsoft Office application that should open that file type, and then click the Always Open with so your documents will open with the right application.