The expr command does arithmetic. It's not just for incrementing a variable in a Bourne shell loop. It's handy for quick integer math on the command line. For instance, if wc tells me a file has 2545 lines and I want to know how many 66-line pages that is:
expr 2545 / 6638
It's 38 pages - actually, 39, because expr rounds down.
Article 45.28 has a complete list of expr math operators.