Large Number

From "Perplexing Lateral Thinking Puzzles" by Paul Sloane & Des MacHale.

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Assume there are approximately 5,000,000,000 (5 billion) people on earth. What would you estimate to be the result if you multiplied together the number of fingers on every person's left hand? (Assume thumbs count as fingers.)

You should have an answer in less than 1 minute.

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Solution to Large Number!