1. Every third fibonacci number is even, so we do a simple loop over all the fibonacci numbers smaller than $$4000000$$ and sum every third one.
2. Every third fibonacci number is even. Since $$F_n = F_{n-3} * 4 + F_{n-6}$$, we initialy calculate $$F_3$$ and $$F_6$$ then $$F_9 = F_6 * 4 + F_3$$, etc…