Thanks for the reply. English is my native language, but I wanted to be 100% sure on this. A lot of English today is colloquial and in my opinion would not be correct in the strictest sense written. We have "explain why", when you change to "explanation" but don't add to/change "why" it seems a bit short. Conversely when you write "explanation as to why" it might sound contrived, but I don't know if it comes from older English (in which case it only seems uppety to us in this age) or is a modern overfiddling with words. I know we can get away with both, but I wanted the official verdict. Do you have any explanation? I was hoping for a textbook answer, so I selected ask a teacher, that might not be necessary, but I would like more on this.