For the first question, the first button you press will have 10 possibilities, from 0-9. For the second button, you will still have 10 possibilities since the same button may be pressed twice. Therefore, you get 10*10=100.
For the second one, the same button may not be pressed twice. For the first button press, you have 10 possibilities, but for the second button, you have 1 less possibility since you aren't allowed to push the button you pressed before. So there will be 9 possibilities. 10*9=90.
