­­­­­The number families that we've worked on before go to the side like this one:

In this number family, what is the greatest (larger) number? What are the lesser (smaller) numbers? Our number family says that 35 is the greatest number, and 14 and 21 are the lesser numbers. Here's the same number family going down:

The lesser number­s are still 14 and 21, and the greatest number is still 35. The greatest number is at the bottom, and the arrow still points to it.

Let us look at this other number family, with one number missing.

And here's the same number family going down.

A lesser number is missing in our number family. So how do we figure out the missing number? We begin with the greatest number. In this case, our greatest number is 43. We subtract the lesser number, 22, from the greatest number, which is 43. This is how we write the subtraction problem:

What is the difference (answer) of the subtraction problem? The other lesser number equals 21. Here is our number family, with our answer, going to the side.

And here's the same number family going down.