There is a machine, which when you put a number card which has written $x$ on it, it will output a number card with $$x^2+10x+20$$ on it. John has a number card on his hand, then repeated the following procedure ten times:
Put the card on his hand in the machine, and get the output.
John found out that he had the number card $0$ on his hand after repeating the procedures.
What number could have been written on John’s card at first?
There is a clever solution with an 'aha' moment.
Problem by me