Jack has a fair coin. He tosses the coin four times. What is the probability that Jack gets heads, then heads, then tails, and then tails?

The answer is 1/16 ...

As you have a 1/2 chance of getting what you want when you flip the coin, to get heads, heads, tails, tails will be 1/2 x 1/2 x 1/2 x 1/2. Working our way down the line, 1/2 x 1/2 = 1/4, 1/4 x 1/2 = 1/8, and 1/8 x 1/2 = 1/16. Jack will get heads, heads, tails, tails 1/16 of the time.