Someone please explain this to me!

Question

I have marked my doubts inside the code:
https://code.sololearn.com/cJntsie2uds2/?ref=app

Answer 1

Since 'func' in this particular case is one of the input parameters given to the defined function 'do_twice', just as 'x' and 'y'. Here, func is some sort of pointer to the also defined function add()
What you actually doing is to tell do_twice what to do by providing the function pointer and on what to do this operation by giving two input arguments, x and y.
In fact the 'add' code is executed three times ((10 + 5) + (10 + 5)). You can use the Debugger to see what code gets executed and what the current values of variables are.