Its about quartiles
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See the attached image for the answers. I've arranged the tiles in the proper order.

This is known as a five number summary.

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Explanation:

The first thing to do is to sort the numbers from smallest to largest

You should get this set {2, 4, 7, 8, 16, 19, 21}

The middle most value is 8. So this is the median or the second quartile.

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Split the data set into two lists like so

L = {2,4,7}

U = {16,19,21}

Everything in set L represents the lower set of values smaller than the median. Set U represents the upper set of values larger than the median.

Find the median of sets L and U to get 4 and 19 respectively. These represent the values of Q1 and Q3

Q1 = first quartile = lower quartile = 4

Q3 = third quartile = upper quartile = 19

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As you can probably guess, the min and max are the smallest and largest values of the set. We can see that min = 2 and max = 21.