In positively skewed distributions, the mean is usually greater than the median, which is always greater than the mode
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.
