You can use bc in bash to do maths calculations with decimals. The man pages can be found:

http://unixhelp.ed.ac.uk/CGI/man-cgi?bc+1

bc -l <<< "20 / ( 20 + 80) * 100 "

The output looks like:

20.00000000000000000000

You can then use this with printf to limit the number of decimals shown: \\The example below shows the results to 3 decimal places. You can replace the 3 with what ever value you wish to place

printf "%.3f\n" $(bc -l <<< "20 / ( 20 + 80) * 100 ")

You can also use bc with variables:

bc -l <<< "($used / ( $free + $used) ) * 100 ")