Determine difference between decimal numbers with 1 and 2 decimals

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Students learn to determine difference between two decimal numbers with one and two decimal places.

5.NBT.B.7

It is useful to be able to determine difference, so you can tell how much more or less you need to reach the other number. You can use this to determine if weight has increased or decreased.

The interactive whiteboard shows two rows of decimal numbers. Each row is missing two decimal numbers. Students must determine what the missing number in the row is. You can show the missing numbers by erasing the grey squares.

Explain to students that when you want to determine difference between two numbers, you look to see how many more or less one number is than the other. When you have one number with one decimal place, and another with two decimal places, it is useful to make the number with one decimal place into a number with two decimal places. You do this by adding a zero in the hundredths place. Show students that they can use a number line to determine the difference by adding to the first number to reach the second number. Do this in three steps. First jump from the decimal number to the nearest whole number, then to the next whole number, and jump to the final decimal portion. Next explain to students that you can also determine difference by making a subtraction problem. You can use turn the numbers 34.56 and 12.4 into the subtraction problem 34.56 - 12.40 and find the difference. Ask students to explain their strategy for subtraction. Next show students how they can solve a story problem, by first determining what kind of math problem is given, determining which numbers are important for the problem, to say or write the problem, and then to solve the problem. Solve one story problem as a class, and then have students solve the second individually or in pairs.

Check that students understand determining difference between decimal numbers with one and two decimal places by asking the following questions:

- How can you determine difference between decimal numbers?

- Which subtraction problem can you make to determine the difference between 53.2 and 68.35?

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

Students are given subtraction problems with decimal numbers that have 1 and 2 decimal places. They are also asked to complete the tables to show the differences between the given numbers. Students are also given story problems which they can choose to solve by creating an addition or subtraction problem.

Repeat the learning goal and refresh why it is important. The interactive whiteboard shows a table with a few weights. Ask students to determine the difference between the different weights. Ask students to explain their strategy. Encourage students to add a zero to the hundredths place to a number with only 1 decimal place to make solving with a second number with 2 decimal places easier.

Students who have difficulty with this learning goal can be supported by the use of a number line. Make sure that they are comfortable jumping from decimal numbers to whole numbers, and then from whole numbers to decimal numbers.

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