# Subtracting Multiples of Ten

Learning how to subtract multiples of 10 from other multiples of 10 is a first grade Common Core math skill: 1.NBT.6. 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.

Prior Learnings

Your students should be familiar with counting from 1 to 100 using 1’s and 10’s, starting from any number. They should also be able to read, write, and represent objects using numbers between 0 and 20 (K.CC.1-3).

Future Learnings

Later on, understanding place values will enable your students to skip-count within 1000 (counting by 5’s, 10’s, and 100’s). They will also be able to read and write numbers by using “base ten numerals, number names, and expanded form” (2.NBT.1-3).

Common Core Standard: 1.NBT.6 - Subtract multiples of 10 by other multiples of 10, within the range 10-90

Students who understand this principle can:

1. Subtract (find the difference) between two multiples of using models, drawings, hundred charts, and number lines.
2. Successfully subtract a multiple of 10 from another multiple of 10.
3. Explain that subtracting by multiples of 10 affects the tens place digit.
4. Explain why the ones place stays the same when subtracting multiples of 10.

Video 1: Subtract Multiples of 10 with Base-Ten Blocks

The video starts by introducing the lesson, explaining the students can write the problem down and/or use base-ten blocks to help them solve the problems. Then the video reminds students of two things:

1. 10 has a 0 in the ones place.
2. Multiples of 10 also have a 0 in the ones place.

Then the video goes into how to subtract multiples of 10. The first problem is 50 - 20 = ?

1. There are 5 ten blocks minus 2 ten blocks.
2. Then, as students might be familiar with adding, the video asks what added to 20 would make 50?
3. Counting up from 20 on a number line, we find that it takes 3 tens to get to 50.
4. If 20 + 30 = 50, then 50 - 20 = 30.

Then the video tries another subtraction problem, this time counting backwards.

1. 70 - 40 = ?
2. To count back, start at 70
3. Count by 10s until you get to 40.
a. 70, 60, 50, 40.
4. How many times did you count back?
a. We counted back 3 times; 3 tens is 30.
5. So, 70 - 40 = 30.

The video ends by reminding students that when subtracting multiples of ten, they can count back by 10 to solve the problem.

Video 2: Word Problem Practice

This short video walks through a simple word problem, demonstrating how to subtract a multiple of 10 by another multiple of 10. Below is the word problem.

“Carl had a bag of 70 marbles, but he tripped on the sidewalk and 40 rolled down a storm drain. How many marbles does Carl have left?”

1. The video draws 70 marbles in groups of 10 to help visualize.
2. When he trips, 40 marbles are lost.
a. 4 groups of ten get erased.
b. How many does Carl have left?
3. The remaining groups are counted.
4. There are 3 groups left, so Carl still has 30 marbles.

The video then summarizes what happened by saying that we started with 70 marbles, subtracted 40, and ended up with 30 for the correct answer.

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