Question

Count the number of strings of length 8 over a = {w, x, y, z} that begins with either w or y and have at least one x.

Answer 1

`1\cdot4^3\cdot2=128` - all starting with w or y

`1\cdot3^3\cdot2=54` - all starting with w or y, not having any x

`128-54=74` - starting with w or y and having at least one x