Answer
$P(15)=0.175$
If we randomly select 20 married people 1000 times, we would expect about 175 trials result in exactly 15 of 20 married people that hide purchases from their mates.
Work Step by Step
$P(x)={}_nC_{x}~p^x~(1-p)^{n-x}$
n = 20
x = 15
p = 80% = 0.8
1 - p = 1 - 0.8 = 0.2
n - x = 20 - 15 = 5
$P(15)={}_{20}C_{15}\times0.8^{15}\times0.2^5=\frac{20!}{15!\times5!}\times0.8^{15}\times0.2^5=\frac{20\times19\times18\times17\times16}{5\times4\times3\times2\times1}\times0.8^{15}\times0.2^5=0.175$
