A spy is trying to open a security door by entering the correct code into a key pad. The
key pad has 10 buttons (for the digits 0; 1; : : : ; 9). How many possible codes does she have
to try at most if...
(a) ...she only knows that the correct code is 4 digits long? I think this is 10!/10C4
(b) ...she knows that the correct code consists of 4 distinct digits?
(c) ...she knows which 4 distinct digits are involved in the correct code?