Answer
$P(at~least~19)=0.0692$
If we randomly select 20 married people 10,000 times, we would expect about 629 trials result in at least 19 of 20 married people that hide purchases from their mates.
Work Step by Step
$P(at~least~19)=P(X\geq19)=P(19)+P(20)={}_{20}C_{19}~0.8^{19}~0.2^{1}+{}_{20}C_{20}~0.8^{20}~0.2^{0}=\frac{20!}{19!\times1!}\times0.8^{19}\times0.2+\frac{20!}{20!\times0!}\times0.8^{20}\times1=20\times0.8^{19}\times0.2+1\times0.8^{20}\times1=0.0692$
