Here's a hint: base10 encoding those 192 bits just means "write the number in decimal". Since 2^192 is about 6E57, that can be done in at most 58 digits.

However, it is not possible to break 192 bits into 58 groups so that each group can be coded in one of those digits. Clearly, some of the bits must end up in more than one of those digits.

base62 is similar to the base10 case, but uses 62 different digits.

If this explanation is insufficient, check http://en.wikipedia.org/wiki/Ascii85. That's for base 85, but the approach is the same.