Unknown in Addition & Subtraction
Determine the unknown whole number in an addition or subtraction equation relating three whole numbers
What a learner can do afterwards
- Solve 8 + ? = 11 and write 3
- Solve 5 = □ − 3 and write 8
- Solve 6 + 6 = □ and write 12
1 · Read
Sometimes an equation has a number hiding. It might look like a question mark or an empty box, like ? or □. Your job is to figure out what number goes there. You already know both sides of an equals sign must match. Use that to find the hidden number.
Dan had some crayons in a box. He gave 3 away and has 5 left. That means □ − 3 = 5. To undo the take-away, add 3 back: 5 + 3 = 8. Dan started with 8 crayons.
Here is the trick: if a part is missing, use the opposite operation to find it. Missing addend? Subtract. Missing start number in a subtraction? Add. Always check your answer by putting it back into the equation.
To find a missing number, undo the operation you see and keep both sides equal.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
18 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.