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How many trees are there in the world and what is the tree-to-people ratio?
(Thanks to A. Müller and C. Loretan, University of Geneva for this question.)
How much total time do we spend bathing each year (including both baths and showers)? Consider your answer for the United States for one year or for one person for their entire life.
Look for the answers online at tpt.aapt.org. Question suggestions are always welcome! For more Fermi questions and answers, see the now available Guesstimation 2.0: Solving Today’s Problems on the Back of a Napkin, by Lawrence Weinstein (Princeton University Press, 2012).
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