Etar, if you are talking about 100 bitcoin challenge competition, all the addresses have the pattern: their 1st (big endian) bit always equals to '1'. You can see for every found private key and present it in binary format, and you can see that the 1st bit always '1' (i mean the 1st of unknown part, without leading zeros).

For example, the key for 80bit address was 0xEA1A5C66DCC11B5AD180, and it is in binary:
11101010000110100101110001100110110111001100000100011011010110101101000110000000

The 1st is '1'.

HardwareCollector absolutely right saying that 80bit address has 79bit search range. And this applies to any other key: n-bit address has (n-1) bit search range.