Compute the following summation using n-1 and k

Question

$$\sum_{k=0}^{n-1} {n-1 \choose k}$$

I have only done this before with integers instead of n-1, and generally the format has an expression after the choose part, so I'm not entirely sure how to begin here.

Answer 1

Since $2^n$ is the summation of $n\choose r$ (taking both the binomial numbers as $1$) then just change the value of $n\to n-1$ and it will end up being $2^{n-1}$.