# A bag contains 2 white marbles and 7 purple marbles. Two marbles are drawn at random. One marble is drawn and not replaced. Then a second…?

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…Then a second marble is drawn.

1. What is the probability of selecting a purple marble and then a white marble?

2. What is the probability of selecting two white marbles?

3. Is there a greater chance of selecting two white marbles in a row or two purple marbles in a row?

Don’t just tell me how to do it, give me some numbers please

• All of these questions depend on the answer of what color marble was drawn first if it’s not replaced…that changes variables in determining the probability of the second event

• 1. The probability of selecting a purple marble is (# of purple marbles) / (total # of marbles).

The probability of selecting a white marble is (# of white marbles) / (total # of marbles).

After you draw the purple marble on the first draw, since you don’t replace it, the total # of marbles has changed.

Probability of both of these things happening = product of the probabilities.

2. Probability of drawing a white marble on the first draw = (# of white marbles) / (total # of marbles).

Probability of drawing a white marble on the second draw = (# of white marbles) / (total # of marbles).

Probability of both of these things happening = product of the probabilities.

After you draw a white marble on the first draw, both the # of white marbles and the total # of marbles for the second draw have changed. So those two probabilities are not the same.

3. Evaluate the probability of two purple marbles the way I did two white marbles.

Compare the two numbers.

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