Intervals
Give a low and a high bound. A narrow range that holds the answer scores most; a miss scores 0.
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How it works
Each item is a sum too big to finish, a Fermi estimate or a probability, and you have about 45 seconds. The score is 1 minus how many orders of magnitude the range spans, so a range of a factor of 2 scores about 0.7. A range scoring 0.5 or more counts as right.
A sample question
Bound the answer: How many times does a human heart beat in years?