Exchange, then find what remains
Use a written subtraction method with value-preserving exchanges.
Explore this lesson →12 practice questions · Optional read aloud · Thinking pad
Let’s understand the idea.
- Keep top minus bottom order in every column; do not swap digits to make a positive digit difference.
- If a place lacks enough units, exchange an available larger unit through adjacent places.
- Each exchange preserves the top amount. Reduce the donor before adding ten units to the next smaller place.
- The prepared top row is still the original amount. After subtracting each bottom digit, every result column must be a digit from 0 to 9.
- Different valid exchange orders can reach the same finished compact record. Check the difference by adding back the subtracted amount.
Something to notice
Have the learner explain every donor and receiver across a zero chain. Pause before removal to check that the top value is unchanged. Then distinguish removing the subtrahend from merely exchanging units. Ask for a fresh independent written example; arbitrary equivalent regroupings are broader than this ready-to-subtract compact record.
Try it. Change it. Explain it.
Open the lesson to use its pictures and controls. In How it works, connect the picture to the idea. In Play & discover, change the quantities and notice what stays the same. In Your turn, answer questions and use the explanation to check your reasoning.
You can ask for help, draw on the thinking pad, or listen to the question and answer options. Progress distinguishes independent work from work completed with help.
Let’s try it →Curriculum connection
Grade 4 · 4.NBT · US Common Core
This lesson is mapped to these standards. A completed activity is practice evidence, not a claim of mastery. Content is checked against the linked official standards and its mathematical models are tested.