The number of 5-digit natural numbers, such that the product of their digits is 36, is K then $\frac{K}{20}$ is.

 $36=2^2\times 3^2$
So, the digits of 5-digit numbers can be
(1,1,1,9,4),(1,1,1,6,6),(1,1,2,2,9),(1,1,3,3,4),(1,1,6,2,3),(1,2,2,3,3)
So, total number of numbers
$=\frac{5!}{3!}+\frac{5!}{3!\times 2!}+3\times \frac{5!}{2!\times 2!}+\frac{5!}{2!}=180$