function [] = chap_5_prob_21()
% 
% Written by:
% -- 
% John L. Weatherwax                2007-04-02
% 
% email: wax@alum.mit.edu
% 
% Please send comments and especially bug reports to the
% above email address.
% 
%-----

% Part (b):
xt = 6*12 + 5;
xb = 6*12; 

zt = (xt-71)/sqrt(6.25); 
zb = (xb-71)/sqrt(6.25); 

( 1 - normcdf( zt ) )/( 1 - normcdf( zb ) )