[lukevalenta]

Here's how you can get that entropy using the Shannon entropy equation from https://en.wikipedia.org/wiki/Entropy_(information_theory).

Like you said, the probability of any digit appearing is 1/30, and there are 10 digits. The probability of a lower- or upper- case letter appearing is 1/78, and there are 26+26 = 52 letters.

Plugging that into the formula for Shannon entropy, we get this:

- 10 * (1/30) * log_2(1/30) - 52 * (1/78) * log_2(1/78) =~ 5.826