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⚡ Addition and Subtraction Facts to 20

Know your addition and subtraction facts to 20 by heart, then stretch them further: if 6 + 4 = 10, then 60 + 40 = 100.

lessons

~15 min

to learn

🔢 Math

subject

Ages 6–12

level

Start the course →

What you’ll learn

Facts to 20 by HeartRecall addition and subtraction facts to 20 fluently, using fact families.Facts to 20, like 7 + 8 = 15, are worth memorising. They travel in families: 7, 8, and 15 give four facts at once. Knowing one member of the family means you nearly know them all, which makes every calculation faster.
Stretch to Bigger NumbersDerive related facts up to 100, like using 6 + 4 = 10 to know 60 + 40 = 100.A known fact stretches to bigger numbers by making everything ten times larger. Because 6 + 4 = 10, then 60 + 40 = 100, and because 15 - 8 = 7, then 150 - 80 = 70. The digits stay the same — you simply scale up.

Questions this course answers

What is 7 + 8?

7 + 8 = 15. Fill the ten: 7 + 3 + 5 = 15.

If 7 + 8 = 15, what is 15 - 8?

They are in the same fact family: take one part away and the other remains.

Which fact belongs to the SAME family as 6 + 9 = 15?

6, 9, and 15 make a family, so 15 - 6 = 9 is part of it.

If 6 + 4 = 10, then 60 + 40 = ?

Everything is ten times bigger: 60 + 40 = 100.

You know 15 - 8 = 7. What is 150 - 80?

Stretch the fact ten times: 150 - 80 = 70.

Grounded in trusted sources

Khan Academy Kids
PBS Kids — early math
National Council of Teachers of Mathematics (NCTM)

Every Wunder lesson is built from real, reputable sources — never invented.

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Wunder is a personalized learn-anything platform — tell it any topic and it builds a beautiful, fact-checked course in minutes, with narration, a knowledge check, and a college-style University track.

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