#! /bin/sh

[ $# -eq 4 ] || {
    echo "Need four integer values a b c d to calculate a*20 - b*2 + c/d"
    exit 1
}
a=$1 b=$2 c=$3 d=$4

result=$(expr $a \* 20 - $b \* 2 + $c / $d)
echo The value of "$a*20 - $b*2 + $c/$d" is $result