In Grade 1 you learned Facts within 20 with tricks like making a ten and using doubles. Now the goal is to just know each fact, the way you know your own name. When facts come fast, your brain has room for bigger problems like $38 + 47$.
Por qué funciona
There are only 100 addition facts with one-digit numbers, from $0 + 0$ to $9 + 9$. Turn-around facts like $8 + 5$ and $5 + 8$ have the same answer, so there are only about half as many to learn. And every subtraction fact hides inside an addition fact: if you know $8 + 5 = 13$, you also know $13 - 5 = 8$ and $13 - 8 = 5$.
Ejemplo resuelto: Make a ten
Find $9 + 6$.
- 9 needs 1 more to make 10.
- Split 6 into 1 and 5.
- $9 + 1 = 10$, and $10 + 5 = 15$.
Respuesta: $9 + 6 = 15$
Ejemplo resuelto: Use a double
Find $7 + 8$.
- 7 and 8 are next-door numbers.
- You know the double $7 + 7 = 14$.
- 8 is one more than 7, so the answer is one more: $15$.
Respuesta: $7 + 8 = 15$
Ejemplo resuelto: Think addition
Find $14 - 6$.
- Ask: 6 plus what makes 14?
- $6 + 8 = 14$.
- So $14 - 6 = 8$.
Respuesta: $14 - 6 = 8$
Idea clave
Know each addition fact well, and you get the subtraction facts for free: if $a + b = c$, then $c - a = b$.
Error común
Counting on and saying the first number again. For $8 + 5$, some people count “8, 9, 10, 11, 12” and get 12. Start at the next number: 9, 10, 11, 12, 13. Quick check: $8 + 5$ is the same as $10 + 3$.
Grown-up tip: Five minutes a day beats an hour once a week. Mix in the facts your child misses, and praise careful thinking more than speed.