Math
posted by Miley .
Suppose that for any integer n,
f(n)= (n1). if n is even;
f(n)= (2n). if n is odd.
If k ∈ N, and f (f (f(k))) = 21, find the sum of the digits in k
I really don't get this means. (∈)

"is an element of "
see:
http://www.glenoaks.edu/facultystaff/FacultyWebSites/JillToner/Documents/NSM105%20%20Math%20for%20El%20Ed/Section%202.1%20%20Set%20Theory.pdf 
f [ f ( f(k) ) ] = 21
f ( f(k) ) = 22
f(k) = 11
k = 12 
so about two digits in twelve