Cards are drawn at random and with replacement from an ordinary deck of 52 cards until a spade appears. what is the probability that at least 4 draws are necessary. Is this idea correct. The probability of picking a all non spades on 1 consecutive draw with replacement is 1/4. 2 consecutive is $(1/2)^2$ and 3 consecutive draw is $ (3/4)^3$. So $ 1 - (3/4)^3$ is the final answer. Is it okay to reason that way? What about without replacement?
What about without replacement
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0$1 - (3/4)^3$ is correct. :) – 2012-09-05