An n-bit binary string is a sequence of length
n over the alphabet {0,1}.
How many n-bit binary strings are there?
How many n-bit binary strings
b1,…,bn are there such that
b1b2≠00? In other words, how many n-bit
binary strings don't begin with 00?
How many n-bit binary strings
b1,…,bn are there such that
b1b2≠00 and b2b3≠11?
How many n-bit binary strings
b1,…,bn are there such that
b1b2≠00 and such that
b2b3≠01?