# Straight up unfair

Level pending

You are bored one day and decide to keep flipping an unfair coin until it lands on tails. It has a $$60\%$$ chance of landing on heads. If you get tails on the $$N$$th flip, the probability that $$N$$ is an integer multiple of 3 can be expressed as $$\frac{a}{b}$$, where $$a$$ and $$b$$ are positive coprime integers. Find $$a+b$$.

×