Unknown in Addition & Subtraction
Determine the unknown whole number in an addition or subtraction equation relating three whole numbers
Ce que l'enfant saura faire ensuite
- Solve 8 + ? = 11 and write 3
- Solve 5 = □ − 3 and write 8
- Solve 6 + 6 = □ and write 12
1 · Lis
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.
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.
À lire aussi
Add and subtract within 20: FAQ
Adding and taking away within 20 is a skill you will use every day. When the numbers are small, you can count on, count back, or make a ten to jump fast. Ten frames and number lines are handy tools that show each step. Pick the tool that feels easiest for the numbers in front of you. Subtraction asks what is left, or what is missing. If 8 + 6 = 14, then 14 take away 8 leaves 6. Fact families stick together, so learning one fact teaches you its partners for free.
2.5: Solving Equations Using the Subtraction and Addition Properties of Equality (Part 1)
Equations with a missing number are puzzles with one hidden piece. Addition and subtraction undo each other, so use the opposite move to reveal it. If something plus nine makes fourteen, take nine from fourteen to find the five. Always test your answer by placing it back into the puzzle. When both sides match, the puzzle is solved.
1.3: Add Whole Numbers (Part 1)
Adding whole numbers means joining counts into one bigger count. Line the digits up by place, start with the ones, and carry tens across when a column passes nine. That carry is just regrouping ten ones as one ten. You can check by adding in a new order, since order does not change the total. Quick recall of facts to 20 makes every bigger sum faster.
Pour aller plus loin
2 · Regarde
En anglaisSans écran
Où se situe ce sujet
À apprendre avant
Liens facultatifs
Ce sujet mène à
Où cela peut mener
Les métiers qui s'appuient sur cette compétence. En suivre un pour voir tout ce sur quoi il repose.
18 questions attendent derrière cette leçon, chacune avec sa réponse expliquée. Chaque réponse nourrit le ciel : les étoiles s'allument à mesure qu'elles sont apprises, et pâlissent quand vient le moment de revenir.