Finding the Missing Addend

Instructor: T.J. Hoogsteen

T.J. is currently a grade 5 teacher and Vice-Principal. He has a master's degree in Educational Administration and is working toward an Ed.D. in Educational Leadership.

In this lesson, you will learn how to find a missing addend. The process taught will show how to solve a problem using subtraction if only one addend and the sum are known.

A Collection

As a child, I loved collecting sports cards. It didn't matter if it was baseball, hockey, or football. I loved them all. One thing about these cards was that you could look on the back and look at the players' career statistics. The other day I was in a store and, for the first time in many years, I bought a pack of hockey cards.

Something weird happened, though, as I was looking at the cards. The back of one of the cards was missing some statistics. The card listed games played, goals, assists, and total points. When looking down the columns, however, it's plain to see that the assists were missing. How was I going to figure out that number?

Missing Assists

Search for the Missing Numbers

First, I looked at the information I had. I knew the number of goals and the total number of points the player scored. I also knew that total number of points is determined by adding together all goals and all assists. So, I wrote the information down from the 2013-14 season to help me figure it out. My equation looked like this:

Missing Addend

After writing it down, I could see that I was missing an addend (a number that is added to another). Then I remembered that adding and subtracting have an inverse relationship, which is just a fancy way of saying that they are opposites of each other. So, as long as I know one addend and the sum (the result of combining the two addends), I could use subtraction to figure out the missing addend.

To do this, you are always going to start with the sum on top, because it is the largest number in the problem, and subtract from the sum the known addend. When adding or subtracting two-digit numbers, it is best to write your problem vertically, like this:

Vertical 1

I get an answer of 33.

To make sure my answer is correct, I take the two addends (the one I knew and the one I just found) and add them together. If I'm correct, they should add up to get the sum (which was 65).

Vertical 2

After adding, I got 65.

I then did the same thing to find out the number of assists in 2014-15. I started with the sum and subtracted the known addend.

Vertcail 3

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