To average [beautiful math coming... please be patient] $\,N\,$ numbers, add them up and divide by [beautiful math coming... please be patient] $\,N\,$.
A convenient way to talk about $\,N\,$ numbers is to use subscript notation.
A subscript is a character (usually, a number or letter) that is written slightly below another
character.
For example, when you look at
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$\,x_3\,$ (read as ‘$\,x$ sub $3\,$’),
the number
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$\,3\,$ is a subscript.
When you look at
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$\,y_b\,$ (read as ‘$\,y$ sub $b\,$’),
the letter
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$\,b\,$ is a subscript.
In subscript notation, we can let
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$\,x_1\,$ denote the first number,
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$\,x_2\,$ denote the second number, and so on.
When more than two numbers are averaged, the concept of balancing point becomes the central idea.
To illustrate the idea, consider finding the average of three
numbers:
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$\,1\,$, $\,4\,$, and $\,6\,$.
Put equalweight pebbles at locations
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$\,1\,$, $\,4\,$ and $\,6\,$ on a number line.
If you think of the number line as a seesaw from a childhood playground,
the support must be placed at the average, for perfect balance!
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$$
\text{average } = \frac{1 + 4 + 6}{3} = \frac{9}{3} = 3
$$
It is clear from the ‘balancing point’ interpretation of the average that
the average of numbers always lies between the greatest number (the one farthest to
the right)
and the least number (the one farthest to the left).
CONCEPT QUESTIONS EXERCISE:
On this exercise, you will not key in your answer.However, you can check to see if your answer is correct. 
PROBLEM TYPES:
