Sreeveluri
But by the 7th marble, we only have 2 marbles to pick from because it is without replacement. If there's only two left, it means atleast one of the blue marbles have already been picked. Now, the the remaining two could be both blue, 1 blue or no blue. How can we say 5/8? We have to do each cases individually, subtract and then add right?
By the 7th draw, the remaining marbles do depend on what happened in the first 6 draws, but we are not given any information about those draws.
Before the drawing starts, the 8 marbles are simply being placed in a random order. Any one of the 8 marbles is equally likely to be in the 7th position, and since 5 of the 8 marbles are red, the probability that the 7th marble is not blue is 5/8.
Consider this: there are 8 cards, 5 spades, and 3 hearts. What is the probability that the first card you pick will be a spade? Clearly 5/8.
Now, the next question: suppose I throw away three cards at random, not telling you which cards I have thrown away. What is the probability NOW that you pick a spade out of the 5 remaining cards? I've just reduced the sample from which you pick, but does the probability change? WHY should it change?
Another question, using the same cards (5 spades and 3 hearts): I say that you can pick one card, but only out of randomly selected 5 cards. What is the probability that the card you pick will be a spade?
There is no difference in ALL the above cases, and the probability will remain 5/8. If it changed, increased (or decreased), it would mean that the probability of picking a heart, on the other hand, decreased (or increased). Why would it?
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