6.SP.A.3
Recognize that a measure of center for a numerical data set summarizes all of its values with a single number, while a measure of variation describes how its values vary with a single number.
Students will learn how to find the mean, median, mode, and range, given a finite list of numbers. Students can go here to practice finding the mean, and here to practice finding the median. Students should also learn the etymology of the word median. After all that, students will learn how to find the mean, median, etc. from stem and leaf plots, dot plots, and frequency tables. Here's a good video for dot plots.
Find the mean and median given a list of numbers. Given the mean and a partial list, find the missing number.
Here's an example problem:
$$7,\ 10,\ ?$$The mean is 8. What's the missing number? Here's the solution.
Have your students try this challenge, then this one. Next, have your students try this one. Then have them try this one and this one. The order doesn't matter here, because the two aforementioned challenges are equally difficult. Then have them try this one, then this one, then this one, then this one. After that, give your students this challenge, and this one too. The two aforementioned challenges can be given in any order, they are equally difficult. Give your students this as the last challenge.