View source for Suppose we flip four coins simultaneously: a penny, a nickel, a dime, and a quarter. What is the probability that the penny and dime both come up the same?

There are <math>2^4=16</math> possible outcomes, since each of the 4 coins can land 2 different ways (heads or tails). There are 2 possibilities for the penny and the dime: either they're both heads or they're both tails. There are also 2 possibilities for the nickel and 2 possibilities for the quarter. So there are <math>2 \times 2 \times 2 = 8</math> successful outcomes, and the probability of success is <math>\dfrac{8}{16} = \boxed{\dfrac{1}{2}}</math>.