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I need to figure out some averages for some javascript I'm writing. The data I have is:

  • I know the average of $a$, $b$ and $c$.
  • I know the value of $c$.
  • I do not know the values of $a$ and $b$.

What I need to know is the average of $a$ and $b$. It must be possible, but I can't figure it out.

2 Answers 2

5

Hint: We have $ 3\cdot\frac{a+b+c}{3}-c=a+b. $

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Let $\frac{a+b+c}{3}=k$ but k is a known value.

Then $a+b+c=3k$. This implies $a+b=3k-c. \cdot\cdot\cdot (1)$

Now, suppose the average of a and b is $m$ an unknown value.

Then $\frac{a+b}{2}=m$Thus, $a+b=2m$. Therefore from $(1)$ $3k-c=2m$. From this $m=\frac{3k-c}{2}$ At last you can put the known values of $k$ and $c$ to get your result.

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    Clear and simple.2011-10-29