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What are some strategies/tips to use when reviewing subtraction with regrouping and subtraction across zeros with 4th graders?
 Math
 35
 Planning
In general: Build procedural fluency off of conceptual understanding and be sure to apply it in context right from the start.
The standard includes use fluent use the standard algorithm. Link the steps in the procedure to conceptual knowledge of place value: ten ones is a ten, ten tens is hundred; Interchange terms regrouping and trading. Build conceptual understanding with physical models first (base ten blocks), show the trading and the answer. Show it pictorially (pictures of the base ten blocks, crossing out and trading). Then show it with numbers. Incorporate visualizing of the blocks when doing the computation with just numbers. Then use some deep questioning. For example, 500142...start with 500, regroup to 4 hundred, 9 tens and 10 ones. Consider asking...are they the same? does it still equal 500? Also, ask "does your answer make sense" and "how do you know?" Also, how can you check subtraction? With the inverse operation  addition! Another idea would be to provide them with a data set (ie height of tall buildings, number of homeruns in career) or opportunity to find their own data set online and have them create subtraction story problems that use that data and solve. They could work in partners. These could be on index cards and swapped with other students. Lastly, procedures tend to need massed practice for initial mastery, so consider some type of daily practice for about 2 weeks.
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I like to use an open number line to teach addition and subtraction. Start with 2digit numbers that are multiples of 10. Establish your endpoints. For example, use an endpoint of 20 and another of 50. Set up your number line so that the distance between 20 and 50 is equivalent to 3 rods (sticks, or groups of 10). Question the students about the multiples of 10 that they would find between 20 and 50 and engage them in a discussion of the placement of those multiples. There will likely be some disagreement among the students. This is your opportunity to address magnitude and to use the base 10 rods to measure an equal distance from 20 to 30, 30 to 40, and 40 to 50, and then label the number line accordingly. The next step is to "hop" from 20 to 30, draw an arc above the number line to connect 20 to 30, and then label that increment as +10. Continue with the rest of the number line. The discussion that takes place will center around how to use the +10s to find the difference between 20 and 50. Can we add 20 + 30 (the combined value of the "hops") and get 50? Can we subtract 30 from 50 and get 20? Will this work with multiples of 100? Could we use different friendly numbers (1, 5, 10, 100) when we make the hops? What determines the size (magnitude) of the hop? Students who struggle with memorizing a traditional algorithm are often successful with this strategy. It's also a useful tool for students to use if they have to cross zero when subtracting with regrouping.
Have you watched this video? It has some interesting tips and strategies for subtraction!https://www.teachingchannel.org/videos/teachingsubtractionmethods