4/8 is equal to 1/2. While this is true of one coin toss, it is not a 50% chance if you toss the coin twice. In tossing the coin twice you have the options of: heads, tails, heads, tails (One head and one tail per coin). Since there are four options, it is a 25% chance you will get heads twice in a row. 25% equals 1/4 which equals 2/8.
