Question

There are two colored marbles and one white marble in a box. Julie mixes the marbles and then Sam draws two marbles at random without replacement. If the two marbles match, Sam wins; otherwise, Julie wins. Does each player have an equal chance of winning? Will adding another white marble give each player an equal chance of winning? What if a game with the same rules was played using three colored marbles and one white marble?

Answer 1

at first there are three marbles for Sam to pick from. the possibility of drawing a colored marble is 2/3. if Sam picks one of the colored marbles, the possibility of therefore (if he'd drawn the colored marble) drawing the second colored marble is 1/2. for the possibility of those happening consequently, we have to multiply the possibilities. 2/3 * 1/2 is 2/6 is 1/3. which means Sam wins in 33.333...% cases. Julie has a better chance of winning.

adding a white marble would change the possibilities to 1/2 and 1/3 consequently, meaning Sam wins in 1/2 * 1/3 = 1/6 = 16.7% cases, which is even less fair.

adding a colored marble would change the possibilities to 3/4 and 2/3 consequently, meaning Sam wins in 3/4 * 2/3 = 6/12 = 50% cases, which finally makes the game fair.