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Explanation:
There are 6 red marbles out of 10+6+3 = 19 total. So 6/19 is the probability of picking one red marble. If that marble is not replaced, then we have 6-1 = 5 red marbles left out of 19-1 = 18 total. The probability of picking another red is 5/18
We have 6/19 as the probability of picking red the first time, then 5/18 as the probability of picking another red marble. The first marble is not put back, and that marble is not replaced.
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In terms of math notation, we can say
A = event of picking red on the first selection
B = event of picking red on the second selection
P(A) = 6/19
P(B|A) = 5/18
P(A and B) = P(A)*P(B|A)
P(A and B) = 6/19 * 5/18