A bit is a 0 or a 1. a bit string is a sequence of digits, all of which are either 0 or 1. for example, there are 8 possible bit
strings of length 3 -- they are 000, 001, 010, 011, 100, 101, 110, and 111. for the problem below, consider a bit string of length 7 -- that is, a sequence of 7 digits, all of which are either 0 and 1
For a string length of 1 there are 2 possible strings (2^1) for a string length of 2 there are 4 possible strings (2^2) for a string length of 3 there are 8 possible strings (2^3) So, for string length of 7 there are 128 possible strings (2^7)
