Solving Problems Involving Missing Numbers in Addition and Subtraction
Primary 4 · Mathematics
Solving Problems Involving Missing Numbers in Addition and Subtraction
Lesson 1 of 4
20 min
Let's begin
Gatete has 350 Frw in his pocket. His aunt gives him some more money. Now he has 1 000 Frw.
How much did his aunt give him? We can write the story as 350 + □ = 1 000. The box hides the number we want to find.
Today you will find hidden numbers in additions.
In an addition, two parts make a whole. In 350 + □ = 1 000, the whole is 1 000. One part is 350. The box is the other part.
To find a missing part, take the part you know away from the whole.
Worked example 1
350 + □ = 1 000
Step 1 □ = 1 000 − 350 = 650.
Step 2 Check: 350 + 650 = 1 000. Yes! Gatete's aunt gave him 650 Frw.
Worked example 2
□ + 2 475 = 6 000
□ = 6 000 − 2 475 = 3 525. Check: 3 525 + 2 475 = 6 000.
You can also count on from the part to the whole. For 350 + □ = 1 000: from 350 to 400 is 50, and from 400 to 1 000 is 600. 50 + 600 = 650.
The sign = means "has the same value as". The two sides must balance, like a level balance.
Worked example 3
45 + □ = 30 + 40
Step 1 Work out the side you know: 30 + 40 = 70.
Step 2 Now 45 + □ = 70, so □ = 70 − 45 = 25.
Careful! Do not just add the numbers you can see. In 350 + □ = 1 000, the answer is not 1 350. The missing part must be smaller than the whole.
Open the Equality Explorer: Basics simulation. Put objects on both sides of the balance and find out what keeps it level.
Tip
Whole take away part. When a part is missing in an addition, subtract the part you know from the whole. Then check by adding.
Work each one in your exercise book first. Then tap Show answer to check.
25 + □ = 60
Answer: 35, because 60 − 25 = 35.
□ + 140 = 500
Answer: 360, because 500 − 140 = 360.
1 250 + □ = 4 000
Answer: 2 750, because 4 000 − 1 250 = 2 750.
38 + □ = 50 + 12
Answer: 24. The right side is 62, and 62 − 38 = 24.
Uwase's book has 200 pages. She has read 86 pages. How many pages are left?
Answer: 114 pages. 86 + □ = 200, so □ = 200 − 86 = 114.
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
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