Determining difference to 100

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Students learn to determine the difference between two numbers with numbers to 100.

2.NBT.B.5

Discuss with students that it is useful to be able to tell the difference between two numbers, you know which one is greater and which is less.

The interactive whiteboard has two images. Ask students to determine the subtraction problem represented by the images. Have students solve the problems. Next show the students a few subtraction problems without visual support. Ask students to determine the difference and ask them to explain their strategy.

Explain to students that if you want to determine the difference between two numbers, you look to see how many more or less the first number is from the second number. Show this with the example of the colored pencils. Count the colored pencils. Ask students to determine what the difference between the two numbers is. Have students practice this with the glue sticks. Next show students that you can also determine the difference of large numbers. Use the colored pencils on the interactive whiteboard to explain how. Then have students practice with the chocolates. Next tell students that they can also use a number line to determine difference by adding to the smaller number to reach the larger number. You can take jumps of 1, 2, 5, 10 etc, whatever works best to reach the number. You can also make a subtraction problem of the two numbers.You can take the numbers 74 and 77 and form the subtraction problem 77 - 74 to find the difference. You can also show subtraction by tens and ones. Show students that they can also use a number line to determine the difference of larger numbers. Show an example. Then indicate that they can also make a subtraction problem. With the numbers 21 and 57 you can make the subtraction problem 57 - 21 to find the difference. Remind students that they can also subtract by tens and ones, and that the larger number becomes the first number of the subtraction problem. Next discuss how to solve a story problem. Explain that students must first determine what kind of math problem is present in the story problem, then determine which numbers are important for the math problem, to say or write the math problem and then to solve. Do a story problem together with the class and then ask students to solve a story problem individually or in pairs.

To check that students understand determining difference from numbers to 100 you can ask the following questions:

- How can you determine difference between two numbers?

- Which subtraction problem would you make to determine the difference between 73 and 29?

- What steps do you take to solve a story problem?

Students first practice determining difference of numbers with visual support. They then are asked to solve abstract and story problems as well.

Discuss with students that it is important to be able to determine the difference between two numbers, so you know which is more or less. Show students a table that they must complete. They must indicate how many cars are left to wash. Then they see two students on the interactive whiteboard throwing darts and telling how many points they earned. The students must determine the difference between the given numbers. Students can solve in pairs and then discuss as a class.

If students have difficulty determining difference, you can give them manipulatives, like MAB blocks to determine the difference between the two numbers. The students can set out blocks for both numbers and then look to see what the difference is.

Optional: MAB-blocks or other manipulatives

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