Suppose that for any integer n,

Question

f(n)= (n-1). 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. (∈)

Answer 1

"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

Answer 2

f [ f ( f(k) ) ] = 21

f ( f(k) ) = 22

f(k) = 11

k = 12

Answer 3

so about two digits in twelve