We've learned already that number families may go either sideways or downw­ard. Let's look at the examples below:

A number family going to the side:

The same number family going down:

Do you still remember how to look for a missing number in a vertical number family? Let's try this example:

In our example, we know one lesser (smaller) number. The missing number is the second lesser number. We figure out the missing number by subtracting. Our problem will look like this:

The difference (answer) equals 29.

Now let's look at the table below. The columns in this table are just like number families that have arrows going down.

The greatest (larger) numbers are at the bottom. The lesser numbers are above the greatest numbers. We'll focus on the first column, starting from the left side of the table. What number is missing in the first column? A lesser number is missing. If we write the subtraction problem, it will look like this:

The difference equals 12.

How about in the second column? What number is missing? This time, the greatest number is missing. We solve for the greatest number by adding the two lesser numbers.

The sum equals 18.

In the third column, a lesser number is again missing. We solve for the missing lesser number this way:

Now we can fill in the table with our answers.