A missing addend is a missing term in an addition sentence that includes at least one other addend and the sum, as shown in the figure below.

In the figure above, the missing addend is 9, since 2 + 9 = 11. Technically, there can be more than one missing addend. In such a case, there is more than one solution to the problem. Most typically however, missing addends are used as a way to teach or practice addition through basic addition facts, which involve the sum of 2 addends whose values usually range from 1-9 or 1-10.

There are various methods for using missing addends to practice addition. One of the most straightforward is completing worksheets of number sentences with missing addends, like the example shown above. A student should first be comfortable with the concept of addition before tackling missing addend problems. In the same way that physical objects or counters can be helpful for learning addition, they can also be used to help a student visualize a missing addend problem. For example, the problem 2 + ? = 9 can be constructed visually as follows:

Using physical objects can be helpful, otherwise just imagine having a pile of objects, like stars, that are not yet part of the addition sentence. Then picture adding these objects to the missing addend portion of the addition sentence, counting the number of objects you need to add in order to make the number of objects on the left side equivalent to the number on the right. In this case, we need to add 7 stars to the 2 we already have in order to get 9, so the missing addend is 7.

Once a student is more comfortable with missing addend problems, word problems can be used to give them practice constructing number sentences with missing addends. Practicing word problems can be helpful because they really test a student's understanding of the concepts involved. Without understanding the concepts, it would be very difficult to convert a word problem into a number sentence they can use to find the solution.

Example

Leila saved \$60. She wants a new bicycle that costs \$100. How much more money does she need to buy the bicycle?

60 + ? = 100

The missing addend is 40. Leila needs \$40 to buy the bicycle.

Missing addends problems can also be used as an introduction to subtraction by rearranging the missing addend problems into subtraction problems with a missing term. For example, instead of 2 + ? = 9, the problem could be written as 9 - ? = 2. The solution, 7, is the same in both cases, demonstrating the relationship between addition and subtraction.