Question

Tommy has a piece of toast that has butter on one side, and he dropped it twice. Both times, it landed with the butter side up. If he drops it 2 more times, what is the probability that it will have landed butter side up a total of 3 times?

Answer 1

Answer: 1/2.

Step-by-step explanation:

First, note that since the previous drops have already happened, their result do not modify the occurrence of future results when the toast is dropped again.

In oder to have a total of three time landing butter side up, exactly, there are two possibilities:

• Either the first next drop lands butter side up and the second next drop lands butter side down, or

• The first next drop lands butter side down and the second next drop lands butter side up.

Since there are two ways of totaling side up 3 times, you have to sum both chances.

1) Next drop butter side up and second drop butter side down:

1/2 × 1/2 = 1/4

2) Next drop butter side down and second one butter side up:

1/2 × 1/2 = 1/4

3) Add both probabilities: 1/4 + 1/4 = 1/2

And that is the answer: 1/2