Operators only possible, if variable defined with decimal
But in beas script you have many functions to work with setvar defined variables (String)
Functions called with %FunctionName(PlaceHolderNames)
Inside the function you don't need to define placeholders with <..>
Example
setvar=a=1
setvar=b=2
setvar=maxValue=%max(a,b)
messagebox=<maxValue>
See