I recently got this question for an engineering manager position. I got it done in an hour and 20 minute with all test cases presented passing. But I don't particularly think this is a good question for senior level candidates since implementing is mostly copy pasting existing functions and renaming them and adding a new memcached VERB.
I'd guess senior level candidates should solve it without copying and pasting, perhaps make sure test cases are present for the behaviour before modifications are made, and then generalize the existing functions. Or, state why adding more functionality on the server side for every type of arithmetic operation would be unnecessary complexity, and instead add support for any type of atomic arithmetic operation with a transaction/UDF approach.
