If the number of 5-element subsets of the set A= {a1 , a2 , …., a20} of 20 distinct elements is k times the number of 5-element subsets containing a4 , then k is

If the number of 5 -element subsets of the set $A=\left\{a_{1}, a_{2}, \ldots ., a_{20}\right\}$ of 20 distinct elements is $k$ times the number of 5-element subsets containing $a_{4}$, then $k$ is