In how many ways can you climb n stairs if each step goes up 1 or 2 stairs? What is the time complexity of computing that count by plain recursion?

In how many ways can you climb n stairs if each step goes up 1 or 2 stairs? What is the time complexity of computing f(n) = f(n-1) + f(n-2) by plain recursion, and of computing f(n) = 2 f(n-1)? Give a 90% confidence interval for how long plain recursion takes to compute f(40) on a 3 GHz processor.