Answer 1

You are calling apply_twice function and passing add_five function name as argument and 10 as well
,apply_twice receives argument and number and returns add_five(add_five(10))
So now that's basically calling add_five function with 10 as argument which returns 15 and then expression us shortened to add_five(15) which in turn returns 20 and so finally 20 is returned to apply_twice function and printed