Question 936452
<pre>
"or" --> add probabilities
"&" = "and" --> multiply probabilities

L = lower
U = upper
G = girl
B = boy

We want 

P(L&G&G or L&B&G or U&G&G or U&B&G) =

P(L&G&G) + P(L&B&G) + P(U&G&G) + P(U&B&G)

P(L) = {{{1/2}}}
P(U) = {{{1/2}}}
P(G from U) = {{{200/500=2/5}}}
P(B from U) = {{{300/500=3/5}}}
P(G from L) = {{{300/700=3/7}}}
P(B from L} = {{{400/700=4/7}}}

P(L&G&G) = (1/2)(3/7)(2/5) =  6/70
P(L&B&G) = (1/2)(4/7)(2/5) =  8/70
P(U*G&G) = (1/2)(2/5)(3/7) =  6/70
P(U*B*G) = (1/2)(3/5)(3/7) =  9/70
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                    TOTAL  = 29/70

Edwin</pre>