How to calculate the geometric mean titer (GMT) of titers from a haemagglutination assay (influenza)?
Typically "&lt;10" is converted to a number that is below the limit of detection; often "9".  After that simply following the instructions you have yields a GMT of 77.9.  If you are using spreadsheet apps such as Excel or Google Sheets, they almost certainly offer "GEOMEAN" functions which will give the number directly.