# The sane Functional equation again

Algebra Level 5

Let $$f: \mathbb{N \to N}$$ be an increasing function such that

$\large f(f(n))= 3n$

If $$x_{i} \in \mathbb {N} \ \forall \ i \in \mathbb{N}$$, $$f(x_{1})= 7$$, $$f(x_{2}) = 12$$, $$f(x_{3})= 21$$, and $$f(x_{4} )= 26$$, find $$x_{1}+ x_{2} + x_{3}+ x_{4}$$.

 Notation: $$\mathbb N$$ denotes the set of natural numbers.

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