Functions asking for user input, with default prompts
Correctness: There are some cases that aren't handled. What happens if the user enters a word instead of a number? What if the user enters 2.1? The program will exit regardless of what character the user enters when exitTimeFn() is called.

I think your overload was a clever solution to a vague instruction. To make it clear that the extra parameter is irrelevant, you could comment out the name. For example:

int askNumberFn(int /*number*/, std::string prompt)
{
    int number;
    std::cout &lt;&lt; prompt;


Because parameters are used to supply data to functions, it doesn't make sense to pass in a value, then immediately re-assign it before use. 

Parameters: I would pass the strings by const reference instead of by value.

Naming: No need for Fn at the end of every function. It's obvious that it's a function from how it's used. The names could also be slightly more descriptive: getNumberFromUser, showExitDialogue, for example.

Commenting: Generally, I'd discourage line-by-line comments explaining what each line of code is doing. It's usually a sign you need to write clearer code. The way you use it in your main function to document your testing has some merit, I think. Look into unit testing for a more scalable/professional way of doing the same thing.

If accidentally using the same input for testing is a concern, I'd just use a different variable completely. A unit testing library would provide better tools to handle this.

While I believe this fulfils the question literally, there is an implied task of reacting reasonably to invalid input. I would test more thoroughly and make sure the edges don't do anything surprising.