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How do I know from which bag I should pick?(for problem 3)

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    for # 3 look at it both ways (green from bag 1 and red from bag 2 or red from bag 2 and green from bag 1) – 2017-01-10

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Let A be the desired event, i.e. that one candy is red and that the other candy is green.

Then A happens in one of two circumstances: 1) you draw a red candy from bag 1 and a green candy from bag 2, or 2) you draw a green candy from the bag 1 and a red candy from bag 2. You simply have to sum the probability of these two events, i.e.

$$ P(A) = P(R_1) P(G_2) + P(G_1) P(R_2) = 0.2 \times 0.2 + 0.1 \times 0.2 = 0.04 + 0.02 = 0.06 $$