Save as .xsl (not .xslx) and any version of Excel should be able to read it.
Save as .ods (OpenDocument Spreadsheet) and open with Microsoft Office Excel 2013 (not 2010 or earlier), since there is common formula support at that level.
Something else to try is to do (2) and put the document on Microsoft SkyDrive. This may allows more download cases for Excel users, although not all features of LibreOffice Calc and Office Excel are preserved this way. It depends on how complicated the rest of the spreadsheet is. It is another, experimental, possibility.