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Matching colour counts in two pairs of jellybeans

Question

A bag holds 66 green and 1010 yellow jellybeans. Four are taken out one after another at random without replacement. What is the probability that the first two drawn and the last two drawn contain the same colours as multisets (order within each pair ignored)? For example, GYYG and GGGG qualify while GGYG does not.

Short answer

135364\tfrac{135}{364}. Either all four beans share a colour, or each pair holds one green and one yellow (in 2×2=42\times2=4 possible orders). Ordered draws give 360360 all-green and 50405040 all-yellow sequences plus 4⋅2700=108004\cdot2700=10800 mixed ones, out of 16⋅15⋅14⋅13=4368016\cdot15\cdot14\cdot13=43680, which simplifies to 135364\tfrac{135}{364}.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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