I understand that the Bitcoin mining problem is to find a string
s (hash of previous block + transaction hashes + nonce) such that
n leading zeros, where
n determines the mining difficulty.
The result of
sha256(s) is a 64-length hexadecimal string. Each position of 64 has 36 possibilities (a-z, 0-9). So, the expected number of guesses would be
The hash of block #485891 is
which has 18 leading zeros. Thus, the expected number of guesses is
36^18 = 10314424798490535546171949056.
This hash was found by Antpool which has a hash power of 1284.32 PH/s. Therefore, the expected time for Antpool to find the hash would be
36^18 / (1284.32 * 10^15) = 8.03104*10^9 s = 255 years.
This time is obviously incorrect. Is there anything I missed out?