Probability of getting 3 blue skittles in even-sized allotments from the same bag

probability

Let's say Skittles come in 3 colors: red, green, and blue. If I pour a bag of 90 Skittles into 10 bowls of 9 Skittles each, the probability of getting >= 3 blue Skittles in any bowl is 1/3. Is the probability of getting >= 3 blue Skittles in all 10 bowls also 1/3?

I would think the probability of all bowls having >= 3 blue Skittles decreases with every additional bowl because of random variance: eg how is the all-bowl probability affected if 1 bowl has, say, 5/9 blue Skittles?

Best Answer

The probability of getting at least $3$ blue skittles in a bowl is $$ 1-\sum_{k=0}^2\frac{\binom{30}{k}\binom{60}{9-k}}{\binom{90}{9}}\approx.632509$$ There are $\binom{30}{k}$ ways to choose $k$ blue skittles, $\binom{60}{9-k}$ ways to choose $9-k$ non-blue skittles, and $\binom{90}{9}$ ways to choose $9$ skittles, so the fraction gives the probability of getting exactly $k$ blue skittles. The probability of getting at least $3$ blue skittles is the same as the probability of not getting $0,\ 1,$ or $2$.

The number of ways to put $3$ blue skittles in each bowl is $$\frac{30!}{(3!)^{10}}\approx$$ because we can line the skittles up in $30!$ ways, put the first $3$ in the first bowl, the next $3$ in the second bowl, and so on. However, the order in which the skittles go into the bowl doesn't matter, so we have to divide by $3!$ in each case. Similarly, there are $$\frac{60!}{(6!)^{10}}$$ ways to put $6$ non-blue skittles in each bowl, and $$\frac{90!}{(9!)^{10}}$$ ways to put $9$ skittles in each bowl, so the probability that there are $3$ blue skittles in each bowl is $$\frac{\binom{9}{3}^{10}}{\binom{90}{30}}\approx.00002598$$

I don't understand the last part. If one of the bowls has $5$ skittles, then the probability of having $3$ skittles in each bowl is $0$.

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