Question 1138074
<b>a) How many different codes are possible if repetition of letters is ​permitted, repetition of numbers is not​ permitted, and the first three entries must all be the same​ letter?</b>
<pr>
26 * 1 * 1 * 26 * 10 * 9 * 8 = 486,720
<pr>
<b>​b) How many different codes are possible if the first letter must be an ​A, ​B, ​C, or D and repetition is not​ permitted?</b>
<pr>
4 * 3 * 2 * 1 * 10 * 9 * 8 = 17,280