# Combinatorial or probabilistic proof of a binomial identity

I'm looking for a combinatorial/probabilistic proof of this identity involving binomial coefficients :

$$\displaystyle\sum_{k=n}^{2n} \binom{k}{n} \times \dfrac{1}{2^k} = 1$$

My try : Multiplying both sides by $2^{2n}$ would render the identity as

$$\displaystyle\sum_{i=0}^n \binom{n+i}{n}\times 2^{n-i} = 2^{2n}$$

That led me to trying to count the number of maps from a set consisting of $2n$ elements to a set of two elements in two ways but I haven't succeeded yet.

Could you please give it a try.

$$\binom{k}n\frac1{2^{k+1}}$$ is the probability that in a sequence of coin tosses, the $(n+1)$-th head is achieved on the $(k+1)$-th toss. Then $$\sum_{k=n}^{2n}\binom{k}n\frac1{2^{k+1}}$$ is the probability that at least $n+1$ heads occur in a sequence of $2n+1$ tosses, etc.