Arithmetic with numeric bash variables?

Question

I have this:

normal=$(find dl -type f | wc -l)
reverse=$(find dlR -type f | wc -l)

where both variables have numbers, but when I try to do something like this:

printf "$normal"+"$reverse"

I dont get the values summed up. What mus I do to add both variables?

Answer 1

 printf "$((normal + reverse))\n"; 

Answer 2

Try with

printf `expr $num1 + $num2`