Answer 1

It's just a name given for the function parameter (for the code example). You can use anything you want there actually, try replacing 'func' with something, then (unless you missed changing one) I suppose the code will run just fine : )

Answer 2

* Why 'func' in argument?
Quoted from the lesson:
"Functions can also be used as arguments of other functions."

* Why 'func' in return? let's first understand some things:

1. The `add` function has 2 parameters <x> & <y> and return x + y.

2. The `do_twice` function has 3 parameters, <func>, <x> and <y>.

Notice `add` is the argument passed for <func> parameter. 5 is the argument for <x> parameter, and 10 is the argument for <y> parameter.

`do_twice` function will call `add` function internally, twice, passing value of <x> and <y> as arguments, receive and use the return value from the two calls to `add` function, and pass them (again) to `add` function, and finally return the result.

return func(func(x, y), func(x, y))
Is equivalent to ...
return add(add(x, y) , add(x, y)) ↓
return add(add(5, 10) , add(5, 10))
↓
return add( 15 , 15 )
↓
And finally ... return 30

Hope this helps, you are welcome to confirm should there be any doubt, me or other friends here will try to help : )