Subtraction from tens numbers with small differences
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General
Students learn to subtract from tens numbers with numbers close to the tens number, which results in small differences.
Common core standard(s)
2.NBT.B.5
Relevance
Discuss with students that it is useful to be able to subtract numbers that lie close to each other. You can then quickly determine, for example, how many cupcakes are left after handing them out in the class.
Introduction
Show a number grid to 100 on the interactive whiteboard and show that some of the numbers are hidden by gifts. Point at a present and ask students to say which number it is covering. Drag the gift to check their answers.
Development
Show a number line. Remind students that they can place the numbers of a subtraction problem on the number line. Point out that the numbers in this subtraction problem are very close together. When numbers are this close together in a subtraction problem, there is an easy method to calculate the difference. Tell students that they must add to the second number to make the first number. That means that they count on from the second number to reach the first number. The number that you counted (or added to the first number) is your difference. Show students a second problem with an accompanying number line. The number line has the numbers of the subtraction problem on it and an arrow. Ask students to add to the second number to reach the first number. Two possible answers are given. Ask students to determine which is correct. There are also problems given in which no possible answer is given, where students must determine the correct answer on their own.
Check that students understand subtraction from a tens number with a small difference by asking the following questions:
- What do you do when you add to the second number in a subtraction problem?
- Calculate the following subtraction problems: 40 - 38 and 70 - 70.
Guided practice
Students are first asked to subtract and select from multiple choice answers. Then they must solve subtraction problems and fill in the answers.
Closing
Discuss with students that it is important to be able to subtract from a tens number with small differences so you can quickly determine the difference. You can use that to determine, for example, how many cupcakes you have left. Show a few problems on the interactive whiteboard. Have students solve the problems on their own sheet of paper (or mini-whiteboard) and then erase the grey boxes on the interactive whiteboard to have students check their work. Finally show the final four subtraction problems. Ask students to solve these problems. The differences match a given color, color those boxes in to reveal an image.
Teaching tips
Students who have difficulty subtracting from a tens number to equal a small difference can make use of the number line to support their calculations. Have students write both numbers on the number line and add to the second number until they reach the first, that number is their difference.
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