you have to use bc and you also need to check argument exist or not

#!/bin/ksh

check(){
    if [ -z "$1" ];then echo "No user input exiting..";exit;fi
       }

check $1

sum=0
n=0
for i in $*
do
    if [ -f $i ]
    then
        while read line
        do
           n=` expr $n + 1 `
           sum=` expr $sum + $line `
        done < $i

        else
           sum=` expr $sum + $i `
           n=` expr $n + 1 `
           fi
done
    average=$(echo "scale=3; $sum/$n" | bc -l )
    echo $average