A lock has two buttons: a "0" button and a "1" button.To open a door you need to push the buttons according to a preset 8-bit se
quence. How many sequences are there? Suppose you press an arbitrary 8-bit sequence; what is the probability that the door opens? If the first try does not succeed in opening the door, you try another number; what is the probability of success?
If you look at both equations they both have 2x in them. This makes it easier to eliminate x. To eliminate x we must subtract the second equation from the first (because 2x - 2x = 0).