# Math

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Suppose that for any integer n,
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. (∈)

• Math -
• Math -

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

f ( f(k) ) = 22

f(k) = 11

k = 12

• Math -

so about two digits in twelve

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