Balls in Urn question

probability

There are 10 red balls, 20 green balls, 30 blue balls in an urn. We take them all out one by one without replacement. What is the probability that by the time we take all the blue balls out there is at least one red ball and one green ball left in the urn?

Best Answer

By symmetry, a particular order is just as likely as its reversal. And in the reversal, we pick at least one red and one green ball before we pick a blue one. But the probability of this is easy to calculate:

The probability that red is the first colour to be picked, followed by green, is

$$\frac{10}{60}\times \frac{20}{50}=\frac{1}{15}$$

And the probability that green is the first colour to be picked, followed by red, is

$$\frac{20}{60}\times \frac{10}{40}=\frac{1}{12}$$

So the answer is $\frac{1}{15}+\frac{1}{12}=\frac{3}{20}$.