Elementary probability theory.
Each position has 1/58 probability to be the desired symbol.
Each position has 1/58 probability to be the desired symbol.
Note: this is not exact correct.
This slightly changes the answer.
If by "not exact correct", you mean potentially orders of magnitudes off (by not understanding how Bitcoin addresses are encoded), than you are correct.
The chance the second digit is "A" is about 1/22. The probability the same position is "1" is 1/256; for it to be "z", 1/1353.
The probability that the 34th digit doesn't exist is about 1/41.
The probability that positions 1-20 are all "1"s is 0.