I have used the following code to Calculate simple intrest and its working fine
intrest=$(( $p * $r * $t / 100 ))
expr: syntax error
intrest=`expr $p * $r * $t / 100`
You need to escape the
intrest=`expr $p \* $r \* $t /100`
This has nothing to do with more than two variables.
intrest=`expr $p * $r`
doesn't work either.