How to add values in a variable in Unix shell scripting?

Question

I have two variables called count1 and count7

count7=0
count7=$(($count7 + $count1))

This shows an error \"expression is not complete; more tok

Answer 1

 echo "$x"
    x=10
    echo "$y"`enter code here`
    y=10
    echo $[$x+$y]

Answer: 20