Question:

A few extraterrestrials arrived. They had two hands, but claimed that 3 + 2 = 11. How many fingers did they have on their two hands? Note that our decimal system (and ten characters: 0, 1, 2, $\dots$, 9) surely has its origin because we have ten fingers. [*Hint*: 11 is in their system. In our decimal system, the result would be written as 5.]

Giancoli, Douglas C., Physics: Principles with Applications, 7th Ed., ©2014. Reprinted by permission of Pearson Education Inc., New York.

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Quick Answer:

2 fingers on each hand