Addition and subtraction facts within 20 (grade 1) · lesson

Count on from the bigger number

💡 Start with the bigger number and count on the smaller one.

For 8 + 3, start at 8. Count 9, 10, 11.

For 5 + 9, start at 9, not 5. Then you only count 5 more.

🧠 How to think about it

Which number is bigger? Put it in your head. Count on with your fingers for the other one.

count on: start at a number and count up

🏡 Try it at home

Roll two dice. Say the bigger number. Count on the dots of the other one.

Make a ten

💡 Fill up a ten first. Then add what is left.

For 8 + 6, take 2 from the 6. Now 8 is 10.

4 is left from the 6. So 10 + 4 = 14.

🧠 How to think about it

9 + 5: 9 needs 1 to make 10. Take 1 from the 5. 4 is left. 10 + 4 = 14.

make a ten: fill a group of ten first

🏡 Try it at home

Use an egg carton with 10 cups. Fill it with 8 coins. Add 5 more. Fill the carton first.

Doubles and near doubles

+

Tap each one to count it

💡 A double adds a number to itself. A near double is one more.

6 + 6 = 12. That is a double.

6 + 7 is one more than 6 + 6. So it is 13.

🧠 How to think about it

See 7 + 8? Think 7 + 7 = 14. 8 is one more than 7, so add 1. It is 15.

double: a number added to itself

🏡 Try it at home

Look for doubles at home. 2 hands, 2 shoes. 5 toes and 5 toes is 10.

Switch the order

+

Tap each one to count it

💡 You can add numbers in any order and get the same total.

3 + 8 = 11 and 8 + 3 = 11.

Same numbers, same total. So pick the easy way.

🧠 How to think about it

4 + 9 looks hard. Switch it to 9 + 4. Start at 9 and count on 4.

total: the answer when you add

🏡 Try it at home

Put 3 forks and 5 spoons in a row. Count. Switch them. Count again. Same?

Take away by adding

💡 To take away, think of the adding fact that matches.

12 − 9 asks: 9 plus what makes 12? 9 + 3 = 12. So 12 − 9 = 3.

Count up from the small number to the big one.

🧠 How to think about it

15 − 8: count up from 8. 8 to 10 is 2. 10 to 15 is 5. 2 + 5 = 7.

difference: the answer when you take away

🏡 Try it at home

Have 12 crackers. Eat 9. Count up from 9 to 12 to find how many are left.