An urn contains 6 green balls and an unknown number, which is ≤ 6, of blue balls. Three balls are drawn successively at random, and not replaced and are all found to be blue.

Determine the probability that a green ball will be drawn at the next draw.

Without an a priori distribution for the distribution of blue balls, there is not a definite way to calculate the probability.

Presumably, we are expected to assume a priori that all values of blue balls between 0 and 6 are equally likely, but this is just an assumption.

After the draw, the number of blue balls are known to be between 3 and 6, but the bayesian probability distribution is no longer equally likely (unless the a priori distribution is very odd).