Answer: 5/18
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Step-by-step explanation
I'll show how to do this without a tree diagram. The next section will show the tree diagram if you are curious.
C = circle
T = triangle
P(CT) = probability of getting circle then triangle, without replacement
P(CT) = P(C and T)
P(CT) = P(C)*P(T given C chosen 1st)
P(CT) = (5/9)*(4/8)
P(CT) = (5*4)/(9*8)
P(CT) = (5*4)/(9*4*2)
P(CT) = 5/(9*2)
P(CT) = 5/18
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In this section, I'll use a tree diagram. See below.
The idea is to start at the top and work your way down.
The fractions correspond to the probabilities of each event. The second row fractions depend on the upper row since replacement is not done.
The A,B,C,D at the bottom represents the four possible outcomes.
Scenario B has us pick a circle 1st and then a triangle next. That results in 5/18 we got earlier.