Shell script to find simple interest

SCRIPT:

#!/bin/bash
echo -n "enter principal : Rs."
read principal
echo -n "enter number of years : "
read year
echo -n "enter rate of interest : "
read interest
simple_interest=`expr "($principal * $year * $interest)/100"|bc`
echo "simple interest = Rs.$simple_interest"

OUTPUT:

sujin@sujin:~/work/shell$ bash SI.sh
enter principal : Rs.2000
enter number of years : 3
enter rate of interest : 8
simple interest = Rs.480