There're a couple parts to your comment. Yes, some third graders could do that no problem, even under the time constraints and pressure. I certainly could - I think I could actually have done it faster in 3rd grade than I could now. It's just like a math test or math competition, and good students have no trouble getting perfect scores on a math test even under intense time constraints (< 1 min/problem) and intense pressure ("If you don't ace this quiz, you'll never get into Harvard and then your life will be wasted.")

The second point of your comment is legit - this sort of time/pressure constraint isn't really representative of the workplace as a whole. Nobody should have someone hanging over them making sure you do your math right in under one minute - you certainly don't at Google. The interviewer probably should've tried to make the candidate feel a bit more at-ease.

But the sort of quick estimation skill necessary for these interview questions is really important. There've been countless meetings when someone (often me) has proposed a solution and I've done some quick mental math calculations only to find out that it just won't work. It's really valuable to get those calculations done in a minute so that you can brainstorm new ideas, rather than waiting until after the meeting when you probably just won't do the math at all.

And this is something that I really wish schools taught more of. Being able to get a quick, mostly-right-but-slightly-wrong answer, and being able to tell what ways your answer is wrong, is probably more important than being able to come up with exact answers or parrot back the formulas you supposedly memorize.