I might have made some mistakes w/ syntax, but you should get the general idea...
if %score% < 20 then msg <Text here>
if %score% < 40 AND %score% > 20 then msg <Text here>
if %score% < 60 AND %score% > 40 then msg <Text here>
However, nothing happens if %score% is exactly 20, 40, or 60. To fix this this code would work:
if %score% <= 20 then msg <Text here>
if %score% <= 40 AND %score% > 20 then msg <Text here>
if %score% <= 60 AND %score% > 40 then msg <Text here>
Also, you have nothing for if score is above 60.