There are 26 letters on a keyboard that would be a possibility to type. The chance of the first letter being unique is 26/26. Since you've already typed one letter, the chance of the second letter being unique is 25/26. The third letter has a 24/26 chance, the fourth letter has a 23/26, and the fifth has a 22/26 chance.
This means that the chance of having 8 different numbers is
[tex] \frac{26x25x24x23x22}{26x26x26x26x26} [/tex]
For the numerator: 26 x 25 x 24 x 23 x 22 = 7,893,600
For the denominator: [tex] 26^{5} [/tex] = 11,881,376
[tex] \frac{7,893,600}{11,881,376} [/tex] = .664367494 = 66%
