Shell script to prepare electricity bill

#Script to prepare Electricity Bill
echo "Enter the name"
read name
echo  "Enter the meter number"
read mno
echo "Enter the current month reading"
read cmr
echo "Enter the last month reading"
read lmr
unit=`expr $cmr - $lmr`
if [ $unit -eq 0 ]
then
       charge=40
elif [ $unit -gt 0 -a $unit -le 100 ]
then
      charge=`expr $unit \* 1`
elif [ $unit -gt 100 -a $unit -le 300 ]
then
     unit=`expr $unit - 100`   
     charge=`expr 100 + $unit`
elif [ $unit -gt 300 ]
then
    unit=`expr $unit - 300`   
    charge=`expr $unit \* 2`
    charge=`expr 300 + $charge`
fi
echo "EB Bill"
echo "---------"
echo "Consumer Name: $name"
echo "Meter Number : $mno"
echo "Units Consumed: `expr $cmr - $lmr`"
echo "Amount to be paid: $charge"


OUTPUT
Safi>sh EBbill.sh
Enter the name
ss
Enter the meter number
3340
Enter the current month reading
500
Enter the last month reading
100

EB Bill
---------
Consumer Name: ss
Meter Number : 3340
Units Consumed: 400
Amount to be paid: 500