# My question is about conditional probability of obtaining $r$ different types out of $k$ different types

I'm suck at Probability, so I need your help on this

let says each chocolate bar contains a card chosen out of $K$ different types with equal probability. A jackpot is offered when r different types of cards are obtained ($K\geqslant r$). I would like to know how to find the probability of obtaining cards of $r$ different types for the first time after buying $n$ bars.

• Does 'for the first time' means that there were only $r-1$ different types of cards among the $n-1$ first bars, and you found a $r$-th type of cards in the $n$-th bar ? – Evargalo Jul 26 '17 at 12:31