Mary’s top book shelf holds five books with the following widths, in centimeters: 6, (1)/(2), 1, 2.5, and 10. What is the average book width, in centimeters?

2010 AMC 10A Problem 1

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Mary’s top book shelf holds five books with the following widths, in centimeters:

6,

\dfrac{1}{2},

1,

2.5,

and

10.

What is the average book width, in centimeters?

1

2

3

4

5

Add the widths before dividing by the number of books

Compute

\left(6+\dfrac12+1+2.5+10\right)\div5

Adding the five widths gives

6+\dfrac12+1+2.5+10=20.

The average is

20\div5=4.

Thus, D is the correct answer.