Show several different ways to supply four missing numbers that are equally spaced and “surround” 2.1 on a number line. An example is shown below.

Add the hash mark values on each of the “number lines” separately. What do you notice? Can you find four additional numbers to complete such a number line so that the sum of all five numbers is 10? Why or why not?

(Source: Mathematics Teaching in the Middle School, Nov-Dec 1995)