What is the probability that if 5 letters are typed, no letters are repeated?

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 

For the numerator: 26 x 25 x 24 x 23 x 22 = 7,893,600
For the denominator:  = 11,881,376

= .664367494 = 66%

It is the top one. the answer is -7. that is smallest