Answer
$P(between~15~and~17)=0.598$
If we randomly select 20 married people 1000 times, we would expect about 598 trials result in between 15 and 17 of 20 married people that hide purchases from their mates.
Work Step by Step
$P(between~15~and~17)=P(15\leq X\leq17)=P(15)+P(16)+P(17)={}_{20}C_{15}\times0.8^{15}\times0.2^{5}+{}_{20}C_{16}\times0.8^{16}\times0.2^{4}+{}_{20}C_{17}\times0.8^{17}\times0.2^{3}=\frac{20!}{15!\times5!}\times0.8^{15}\times0.2^5+\frac{20!}{16!\times4!}\times0.8^{16}\times0.2^4+\frac{20!}{17!\times3!}\times0.8^{17}\times0.2^3=15504\times0.8^{15}\times0.2^5+4845\times0.8^{16}\times0.4^4+1140\times0.8^{17}\times0.2^3=0.598$
