3
Number stories and estimates
Learner’s Book 1: Unit 13, pp. 194–196
GoalWrite a number sentence for a word problem, estimate the answer and then solve it.
New words
word problem · a maths question told as a storynumber sentence · numbers and signs that tell the storyestimate · a sensible guess
Explanation
A word problem is a maths question told as a story. Read it slowly. Ask: do things join together, or are some taken away? If more come, add. If some go away, subtract. Then write a number sentence. Before you solve it, make an estimate. For 9 + 8, both numbers are close to 10, so the answer should be close to 20. Then solve: 9 + 8 = 17. 17 is close to 20, so our estimate was sensible.
| story | number sentence |
|---|---|
| more come | + |
| some go away | − |
Worked examples
A shelf has 7 books and the teacher puts on 8 more. How many books now? 7 + 8 = 15.
A basket has 15 apples. 6 are eaten at lunch. How many are left? 15 − 6 = 9.
Estimate first: 9 + 8 is close to 10 + 10, so about 20. Then solve on the line: 9 + 8 = 17.
Class activity
Story makers: pick two numbers up to 20, such as 6 and 9. Make up a short story that adds them, and another story that subtracts them. Write the number sentences. Swap stories with a partner and solve each other’s.
Practice
1
Nodira picks 8 red flowers and 7 yellow flowers. How many flowers? Write the number sentence.
8 + 7 = 15
2
There were 18 grapes in a bowl. Sara ate 5. Write the number sentence and solve it.
18 − 5 = 13
3
Estimate: is 12 + 7 closer to 10 or to 20? Then solve it.
Closer to 20. 12 + 7 = 19.
4
Kenji had 17 toy cars and gave 4 away. Write the number sentence and solve it.
17 − 4 = 13