2006 iTest Problems/Problem 2

Problem

Find the harmonic mean of 10 and 20.

$\mathrm{(A)}\, 15\quad\mathrm{(B)}\, \frac{40}{3}$

Solution

The harmonic mean of two numbers is the reciprocal of the mean of the reciprocals of the two numbers. The reciprocals of $10$ and $20$ are $\tfrac1{10}$ and $\tfrac1{20}$ and the mean of the two is $\tfrac3{40}$ , so the harmonic mean is $\boxed{\textbf{(B) } \tfrac{40}{3}}$

2006 iTest (Problems, Answer Key)
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2006 iTest Problems/Problem 2

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