Question

A coin is biased in such a way that on each toss the probability of heads is $\dfrac{2}{3}$ and the probability of tails is $\dfrac{1}{3}.$ The outcomes of the tosses are independent. A player has the choice of playing Game A or Game B. In Game A she tosses the coin three times and wins if all three outcomes are the same. In Game B she tosses the coin four times and wins if both the outcomes of the first and second tosses are the same and the outcomes of the third and fourth tosses are the same. How do the chances of winning Game A compare to the chances of winning Game B?

The probability of winning Game A is $\dfrac{4}{81}$ less than the probability of winning Game B.

The probability of winning Game A is $\dfrac{2}{81}$ less than the probability of winning Game B.

The probabilities are the same.

The probability of winning Game A is $\dfrac{2}{81}$ greater than the probability of winning Game B.

The probability of winning Game A is $\dfrac{4}{81}$ greater than the probability of winning Game B.

Solution:

Let $p = \dfrac23.$ Game A is won when all three tosses match: $p^3 + (1-p)^3.$ Game B needs the first pair to match and the second pair to match, each with probability $p^2 + (1-p)^2,$ so the win probability is $\bigl(p^2 + (1-p)^2\bigr)^2.$ With $p = \tfrac23,$ Game A gives $\left(\tfrac23\right)^3 + \left(\tfrac13\right)^3 = \tfrac{9}{27} = \tfrac13,$ and Game B gives $\left(\tfrac49 + \tfrac19\right)^2 = \left(\tfrac59\right)^2 = \tfrac{25}{81}.$ The difference is $\tfrac{27}{81} - \tfrac{25}{81} = \tfrac{2}{81},$ so Game A is $\tfrac{2}{81}$ more likely.

Thus, the correct answer is D.

2017 AMC 12B Problem 17

Solution:

Problem 17 in Other Years