One-step and Two-step Problems Using Subtraction Strategy

Introduction

Jake has 34 stickers; Sara has 12 fewer stickers than Jake, and Blake has 58 stickers. How many more stickers are there with Blake than Jake? How many stickers are there with Sara? How many fewer stickers are there with Sara than with Blake?

Answers:

Stickers with Jake = 34

Sara has 12 stickers fewer than Jake

Stickers with Sara = 34 – 12 = 22

Stickers with Blake = 58

The number of stickers with Blake than Jake = 58 – 34 = 24

The number of fewer stickers with Sara than Blake = 58 – 22 = 36

Question:

There are some students in the class. 49 more students joined them. Now there are 78 students in the class. How many students are there in the class at the start?

How can you find the answer?

Solution:

Some students are in the class.

49 students joined them.

Now, there are 78 students.

To find the students in the class at the start:

We must solve 78 – 49.

Break apart 49 into 40 and 9

So, 78 – 49 = 29

There are 29 students in the class at the start.

Let us see some more examples.

Example:

Kim has 9 candies. Kim’s uncle gave him 15 more candies. Now, he gave 8 candies to his sister Linda.

How many candies are there with Kim now?

Solution:

We can use bar diagrams to solve the problem.

Step 1:

Find the number of candies Kim has after he received candies from his uncle.

Kim has 24 candies.

Step 2:

Find the number of candies after Kim gave 8 candies to Linda.

Kim has 16 candies.

Check your answer:

Kim has 9 candies.

He received 15 candies from his uncle.

He gave 8 candies to Linda.

Now he has 16 candies.

9 + 15 = 24

24 – 8 = 16.

The answer is verified.

Problem Solving:

Example 1:

Jake sticks 84 stickers. Sara sticks 48 stickers.

Jill says that Sara stuck 34 fewer stickers than Jake.

Jill thinks it is easier to subtract 84 – 50 than 84 – 48. So, he subtracts 84 – 50 and says 34.

Will you agree or disagree with Jill’s thinking?

Solution:

How can we decide whether we can agree with Jill or not?

Use compensation to subtract.

First, add 2 to 48 then use mental math to find the difference then add 2 to get the final answer.

Sara has 36 fewer stickers than Jake.

We disagree with Jill.

Example 2:

Duane has 64 puzzle pieces. Jan has 38 Puzzle pieces. Ron says Duane has 26 more puzzle pieces than Jan. Ron says he found 64 – 38 using an open number line. He added up 2 and 24 more from 38 and got 64

Is Ron’s reasoning, correct?

Do you agree?

Solution:

Duane has 64 puzzle pieces.

Jan has 38 puzzle pieces.

Ron says Duane has 26 more puzzle pieces than Jan.

Ron says he found 64 – 38 using an open number line.

He added up 2 and 24 more from 38 and got 64

Ron’s reasoning is correct.

Exercise

(1-6) Use any strategy and subtract.

1. 94 – 15.
2. 88 – 26.
3. 72 – 38.
4. 67 – 42.
5. 55 – 28.
6. 47 – 19.
7. Use an open number line to add up and subtract 64 – 42.
8. Use a bar diagram to subtract 44 – 21.

(9-10) John has 24 candies. Ron has 27 more candies than John. John has 12 fewer candies than Duane.

9. How many candies are there with Ron?

10. Yoshi says Duane has 12 candies. Instead of subtracting 24 -12, he subtracts 24 -10 then subtracts 2 more. Is Yoshi’s reasoning, correct? Do you agree with Yoshi?

Concept Map:

What we Have Learnt

• One step and two step problems
• Problems on subtractions within 100 using any strategy