Question 381557
{{{BG}}}
{{{BB}}}
{{{GG}}}
{{{GB}}}
Those are the 4 possible combinations.
{{{P(GG)=1/4}}}, Good.
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At least one girl, there are 3 outcomes with at least one girl. 
Two girls is also at least one girl.
{{{P(1 G)=3/4}}}
You can also look at it this way.
At least one girl means no boys.
P(all boys)+P(not all boys)={{{1}}}
P(at least 1 girl)={{{1}}}- P(all boys)
P(at least 1 girl)={{{1-P(BB)}}}
P(at least 1 girl)={{{1-1/4}}}
P(at least 1 girl)={{{3/4}}}
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{{{P(GB)=1/4}}}, Good.