In the first part of this article, you learned how to use base ten blocks to show numbers and add them. When these two skills are mastered, it's time to move on to subtraction, which is many kids' worst fear. You may have heard that subtraction is just addition backward. It can be hard to do on paper, but base ten blocks can make it easy.
Remember that there are four different kinds of base ten blocks: cubes (ones), rods (tens), flats (hundreds), and blocks (thousands). Groups of ten base ten blocks can be rearranged or traded for equivalent amounts of other bases ten blocks. For example, ten cubes can be traded for one rod because both are worth ten. It is helpful to know how to trade down rods, flats, and blocks when doing subtraction. Trading down means changing blocks with a larger place value into blocks with a smaller place value. For example, you can trade one flat for ten rods, since both are worth 100. Before we talk about how to do subtraction, let's go over some words. Minuend is the amount you are taking away. subtract - The amount you are taking away. The answer to the question.
In the equation 234 - 187 = 47, 234 is the "minuend," 187 is the "subtrahend," and 47 is the "difference." Most people don't care about the words "minuend" and "subtrahend," but they help explain how to do subtraction with base-ten blocks.
First, use base ten blocks to show the minuend. Try to keep the blocks in order from biggest to smallest. This will help you later when you have to use paper and pencil. Take out enough blocks from the minuend piles to make the subtrahend. If there aren't enough blocks, trade some of the larger place value blocks until there are enough smaller place value blocks to remove. The difference is shown by the piles that are left after the subtrahend is taken away.
Start by showing the number 234 with 2 flats, 3 rods, and 4 cubes. The goal is to take out 187, which is 1 flat, 8 rods, and 7 cubes. Taking out one flat is easy, but taking out 8 rods and 7 cubes when there are only 3 rods and 4 cubes is hard! Trade one flat for 10 rods and one rod for 10 cubes to solve this problem. One flat, 12 rods and 14 cubes would be the result. If you took away the subtrahend, which is 1 flat, 8 rods, and 7 cubes, you'd be left with no flats, 4 rods, and 7 cubes. After taking out the subtrahend, the difference is whatever is left, so the difference is 47.
Beginners should start with subtraction problems that don't involve trading. For example, 1954 - 1831 doesn't need to be traded because there are enough blocks in the minuend to remove the subtrahend. Questions with zeros can be a bit of a challenge for students who are more skilled. For example, to get from 4000 to 3657, you would have to trade blocks starting with 4. There are thousands of free math worksheets, including subtraction problems with no regrouping (trading). One of the nice things about this website is that there are answer keys, so students can see how they did.
Students learn how to do subtraction on a conceptual level after doing it enough times. This makes it easier for them to use pencil and paper methods in the future. When students only use paper and pencil to learn subtraction, they don't always get a good grasp of how it works and are less likely to be able to spot mistakes in their work.
Base-ten blocks can be used for more than just adding and taking away whole numbers. In part III of this series, we'll look at some other ways to use base ten blocks.
