Your question is Back-of-Envelope Finger Math. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Assume there are 7,000,000,000 (7 billion) people on Earth. If you multiply together the number of fingers on every person's left hands, what would you estimate to be the result?