SOLUTION: Probability of getting at least 65 girls in 100 births

This is a binomial probability.  We use the normal approximation
of the binomial:




Since the probability we want is P(x65), for
proper estimation we use P(x64.5). 

We find the z-score for x=64.5



We find 2.9 in the leftmost column for z and 0 across the top
which means we want the entry just to the right of 2.9 which 
is .4981.  This is the area between z=0 and z=2.90, so to get
the area right of that we must subtract 0.4981 from 0.5, and
get .0019.

If we use a TI-83 or TI-84 we get 

1-binomcdf(100,.5,64)=1-.9982411791=.0017588209

which rounds to .0018 rather than .0019 but we can expect such 
small differences between answers found using a graphing 
calculator and those found using a normal table.

Edwin