Solving Problems Involving Missing Numbers in Addition and Subtraction
Primary 4 · Mathematics
Solving Problems Involving Missing Numbers in Addition and Subtraction
Lesson 2 of 4
20 min
Let's begin
Keza went to the market with some money. She spent 1 200 Frw on tomatoes and came home with 800 Frw.
How much money did she take to the market? We can write □ − 1 200 = 800. This time the missing number is at the start!
Today you will find missing numbers in subtractions.
1. The answer is missing: 900 − 350 = □. Just subtract: □ = 550.
2. The whole is missing: □ − 1 200 = 800.
Keza's money was the whole. It was split into the part she spent and the part left. So add the parts: □ = 1 200 + 800 = 2 000. Keza took 2 000 Frw.
Check: 2 000 − 1 200 = 800. Yes!
3. The number taken away is missing: 900 − □ = 350.
900 is the whole and 350 is what is left. The box is the part taken away: □ = 900 − 350 = 550. Check: 900 − 550 = 350.
| Where is the box? | What to do | Example |
|---|---|---|
| the answer | subtract | 900 − 350 = □, so □ = 550 |
| the first number (the whole) | add the other two | □ − 1 200 = 800, so □ = 2 000 |
| the number taken away | subtract | 900 − □ = 350, so □ = 550 |
Careful! When the first number is missing, you must add. Many children do 1 200 − 800 = 400. But 400 − 1 200 cannot leave 800! Always check.
Tip
Missing whole? Add. Missing part? Subtract. Draw a bar. If the box is the whole bar, add the parts. If the box is a part, take the other part away from the whole.
Work each one in your exercise book first. Then tap Show answer to check.
700 − 260 = □
Answer: 440.
□ − 45 = 80
Answer: 125. The whole is missing, so add: 80 + 45 = 125.
500 − □ = 320
Answer: 180. The part taken away is missing: 500 − 320 = 180.
□ − 2 350 = 1 650
Answer: 4 000. Add: 1 650 + 2 350 = 4 000.
A bus had some passengers. At Muhanga, 18 got off and 27 stayed on. How many passengers were on the bus at first?
Answer: 45. □ − 18 = 27, so □ = 27 + 18 = 45.
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
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