Question 187364
A couple plan to have exactly three children. 
(a) Construct a tree diagram and list the sample space.
1st column: b,g
2nd column: b,g;b,g
3rd column: b,g;b,g;b,g
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(b) Find the probability that the family has at least two girls
ggb ; gbg ; bgg ; ggg
P(at least 2 girls)  4/8 = 1/2
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Cheers,
Stan H.