If someone is looking for a more comprehensive explanation: I found this article How to Evaluate Arithmetic Expressions in Bash (by Baeldung) very helpful.

My personal favorite is the Arithmetic Expansion ($(( 1+1)) / $(( a+b ))), but there's also the alternative approaches like the let and the expr command explained, as well as the bc command for floating-point operations.

For details on what you can do with pretty much all of those approaches, see the Shell Arithmetic page from the official Bash Reference Manual. There's the list of supported operators.