What are the odds that after hitting shuffle twice on a 155 unique songs playlist on spotify, 3 of the 6 first songs are the same on both shuffle.

combinationsprobability

Imagine you have $155$ unique songs in a playlist on Spotify. You hit shuffle. Out of the first $6$ songs, you have $3$ particular songs that we'll name "song A", "song B", and "song C". The order in which these $3$ songs show up within those first $6$ songs doesn't matter. Then, you hit shuffle again and get another set of $6$ songs. $3$ of those $6$ songs are once again A, B, and C. What are the odds of this happening?

I'm kinda stuck trying to math this. I know that you have

$$\frac{155!}{6!(155-6)!}$$

different ways to select $6$ songs and that would be a denominator for a probability "P". You would then get a numerator and multiply $P$ by itself ($P^2$) because we shuffle $2$ times. Anyways, I think that's what I need to do. The numerator is where I get stuck. Not sure how to calculate it. Anyone can help? Am I on the right track?

Best Answer

For $3$ particular songs to appear in the first $6$ songs in a shuffle, all we need is to choose rest of the $3$ songs from the remaining $152$ songs.

If you knew before the first shuffle that you were looking for $3$ particular songs - $A, B, C$, then the probability that you get these $3$ songs in both the shuffles is,

$\left[\displaystyle {152 \choose 3}/{155 \choose 6}\right]^2$

But if you marked $3$ particular songs out of $6$ as $A, B, C$ after the first shuffle (which is what the question seems to suggest), then it is just,

$\displaystyle {152 \choose 3}/{155 \choose 6}$

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