The Normal approximation to the binomial
is as follows
mean=13(.5)=6.5
variance=6.5(.5)=3.25
Pr[fewer than 4]=
Pr[N<=3] Apply the continuity correction
Pr[N<=3.5]~=Pr[Z<3.5-6.5/sqrt(3.25)]=
Pr[Z<-1.66]=0.048457226
I hope my answer has come to your help. God bless and have a nice day ahead!