Learning to confidently add and subtract 10 or 100 from a number between 100-900 is a second grade, Common Core math skill: 2.NBT.8. Below we show two videos that demonstrate this standard. Then, we provide a breakdown of the specific steps in the videos to help you teach your class.
Your students should be familiar with the first grade skill of counting up to 120 starting from any number below 120 (1.NBT.1), this skill helps them understand greater or less than values. The second grade skill is also closely linked to the first grade skill of understanding place values (ones and tens) in two-digit numbers (1.NBT.2).
Comparing large numbers as greater than, less than, and equal to will help your students understand future concepts in third grade. In third grade, your students will learn how to interpret the products of whole numbers (i.e. 8 x 3 is the same as 8 groups of 3 objects each) (3.OA.1). They will also learn to use multiplication and division within 100 while solving word problems in situations that involve “equal groups, arrays, and measurement quantities” (3.OA.3).
Common Core Standard: 2.NBT.8 - Mentally add and subtract 10 or 100 to a given number between 100-900
Students who understand this principle can:
2 Videos to Help You Teach Common Core Standard: 2.NBT.8
Below we provide and breakdown two videos to help you teach your students this standard.
Video 1: What’s 100 Points More or Less?
The video begins by introducing a word problem: “Alice’s teacher gives her 10 points for every A she earns on a test. Alice has 110 points already. [That’s a lot of tests!] How many points will she have if she earns 100 more?”
If Alice receives 100 more points, she will have 210 in total.
Now, assume the question asked if Alice lost 100 points. What’s 110 - 100?
So if Alice lost 100 points, she’d only have 10 left.
Video 2: Exercising while Subtracting!
This video offers students a chance to move and exercise while learning to mentally subtract 10 from a number. The concept can easily be adapted for addition as well!
The video starts by explaining what the students will do and the rules of the game. Students will do an exercise depending on the number inside the box. For instance, 90 - 10 = 80; 8 is in the box, so they would do the exercise 8 times, not 80.
Below is the subtraction problems and the exercise related to them:
That’s it! Hopefully this video helps inspire some fun ideas for classroom learning!
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*Information on standards is gathered from The New Mexico Public Education Department's New Mexico Instructional Scope for Mathematics and the Common Core website.