 Addition and subtraction to 20 using related problems

# Addition and subtraction to 20 using related problems

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## General

Students learn to add and subtract numbers to 20 in which they are asked to add or subtract a certain number more or less to a set of problems. They complete a table with the related problems.

1.OA.C.6

## Relevance

Discuss with students that it is useful to be able to add or subtract with one more or one less. You can use this to solve a set of math problems.

## Introduction

Give students a pop quiz. Ask them to solve as many problems as possible in the given time. Discuss the problems and give the answers. Then give students a few story problems and ask them to determine what the math problem should be. Ask them to explain their strategy and then ask students to solve the created math problems.

## Development

Show students the cake with candles on the interactive whiteboard. Tell students that there are four candles on the cake, and that one more is being added. Together you now have five candles. Discuss the other math problems shown by the cake with candles. Ask students if they notice anything about the math problems. They should notice that one more candle is being added each time. There aren't always images or pictures to count when adding. Sometimes you are only given a number, and told how many are added. Solve the first problem and then you count one more as long as the same amount of candles is added. Tell students that they have to look carefully at how many are added. There are problems with one more, with two more, three more, four more, or even more! Tell students that you always need to solve the first problem to be able to easily solve the rest of the problems in the set. Show students the table and show students how to fill in the missing numbers of the table. Ask students to explain their strategy.
Next show students a cake with candles and determine, together with the students, that there are 14 candles on the cake. One candle has been blown out. That means that you still have 13 candles burning. Discuss the other math problems shown by the cake with candles being blown out or lit. Ask students if they notice anything about the math problems. They should notice that there is one more candle on the cake each time, but that the same number of candles is blown out each time. Tell students that they must carefully look at how many less it is. Point out to students that even though they are subtracting, since the number they are subtracting from becomes larger each time, the difference also becomes larger. Some problems have one less, two less, three less, four less, and even larger numbers less! Tell students that they must always solve the first problem to be able to easily solve the rest of the problems in the set. Show students the table and show students how to fill in the missing numbers of the table. Ask students to explain their strategy.
Finally show students the table in which students are asked to solve both more and less. Ask students to write their answers on a mini-whiteboard (or sheet of paper) and hold them up to check their work.

Check that students are able to add and subtract numbers to 20 using related problems by asking the following questions:
- What is the first step to take to be able to easily complete the table?
- If you know the answer to the first problem, do you also know how much more to add to the next problems in the set? How much is it?

## Guided practice

Students are asked to complete tables with more and to complete tables with less. They are then challenged to complete tables with more and less. Remind students that the solving the first problem is most important.

## Closing

Discuss with students that it is important to be able to add and subtract with numbers to 20 using related problems, so you can easily solve a set of math problems without needing to recalculate each problem. Show the table on the interactive whiteboard and ask students to check them. They should correct the tables that are incorrect and explain how they knew that they were incorrect. Check that students solved the first problem and then added to or took away from the answers rather than recalculating each problem.

## Teaching tips

Students who have difficulty with adding and subtracting with a set number more or less can make use of MAB blocks or other manipulatives. Have them count out the total number of blocks and then show students a problem with one/two/three more. The students must add one/two/three blocks to the existing problem that they have. Practice this with less as well.

## Instruction materials

Optional: MAB blocks or other manipulatives. 