You have to get the right letter and the right number to get the combination correct. There are 26 letters that you could pick from and there are 5 numbers (1,3,5,7,9). They both must be correct.
1/26 chance you get the right letter, 1/5 chance you get the right number.
1/26 * 1/5 = 1/(26*5) = 1/130
The chances of getting it right the first try is 1 in 130.