1
Counting back in ones
Learner’s Book 1: Unit 13, pp. 188–190
GoalSubtract by counting back in ones along a number line.
New words
subtract · take awaycount back · start at a number and say the numbers that come before itleft · how many are not taken away
Explanation
Sometimes some things are taken away, and we want to know how many are left. We subtract. To subtract, we count back. Start at the first number and count back the second number, one at a time. On the number line we draw a small jump to the left for each number we say. To find 12 − 5, start at 12. Count back 5: 11, 10, 9, 8, 7. We land on 7, so 12 − 5 = 7.
Worked examples
Nodira has 16 grapes and eats 3. Start at 16 and count back: 15, 14, 13. She has 13 grapes left. So 16 − 3 = 13.
To take 4 from 10, start at 10 and count back: 9, 8, 7, 6. So 10 − 4 = 6.
We can show subtracting with a picture too. Leo has 8 cherries and eats 3. The 3 crossed-out cherries are gone. 5 are left. 8 − 3 = 5.
Class activity
Hopping back: draw a number line from 0 to 20 on the ground or on paper. Start on a number between 10 and 20. Roll a dice and hop backwards that many steps, saying each number. Where do you land? Say the subtraction.
Practice
1
What is 14 − 3? Start at 14 and count back in ones.
11
2
What is 8 − 5? Start at 8 and count back in ones.
3
3
What is 17 − 2? Count back from 17.
15
4
Tom has 18 stickers and gives away 4. How many stickers does he have left?
14 stickers (18 − 4)