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Because he was using a '%' char in one of the printf(), which would have thrown an error as he wasn't using it appropriately. Advising him to use println() was far more efficient that explaining the...

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Yes all of them; each comma denotes a new input argument, and println() only accepts one String input.

e.g. public void addNumbers(int numOne, int numTwo) << 2 inputs
public void...

That issue is because you're attempting to add multiple parameters into the println() method, which only accepts 1 String.
Replace the comma with a '+'

I assume he will have already replaced the prinf() with println() as he would have run into runtime errors using % in the code.
Not that you should have known that ofcourse.

I can't seem to find anything wrong with your code.
Try and recompile / build your project, in-case you're using outdated class files.

Note though; " 10% " within a printf() method is going to...