I have a question about a probability problem located here:
And quoted here:
“If you were trying to collect 6 baseball cards that came in packets of cheese puffs, assuming they are distributed evenly how many packets of cheese puffs would you expect to buy before you have all 6 cards?”
I was in agreement with the first part (Case I), in that one is guaranteed to get some card on the first packet.
However, they lost me at part II (Case II), with their odds calculation on that step. I would have assumed since we still needed 5 cards of the 6 cards total, with each being equally likely, and given that we already have one of the cards, the odds of getting a card we needed would have been 5/6. However, they say 4/5. (Also, they have 4/5 = 125/100, which sure looks fishy to me!)
I would have further guessed 4/6, then 3/6, then 2/6 and finally 1/6 for the last card. I also figured that the reciprocal of these summed is the number of cheese puff packets one would have to buy to collect all 6 cards, or 13.7 packets total
Can somebody point out where I am going astray with my method? Or, is this a rare case where I'm okay and the reference is wrong?