Solving Problems Involving Missing Numbers in Addition and Subtraction
Primary 4 · Mathematics
Solving Problems Involving Missing Numbers in Addition and Subtraction
Lesson 4 of 4
20 min
Let's begin
Habimana's family grows avocados. On Monday morning they had some avocados in the store. During the day they sold 1 450. In the evening they picked 800 more. At night they counted 2 600.
How many avocados did they have in the morning? Today you will turn a story into a number sentence with a box, solve it and check it.
Worked example 1
Keza wants shoes that cost 5 000 Frw. She has 3 250 Frw. How much more does she need?
Sentence: 3 250 + □ = 5 000. Solve: □ = 5 000 − 3 250 = 1 750 Frw. Check: 3 250 + 1 750 = 5 000.
Careful! The word "more" does not always mean add. Here the sentence has +, but you find the box by subtracting.
Worked example 2
Mutesi had some mangoes. She gave 36 to her neighbours and has 48 left. How many did she have?
Sentence: □ − 36 = 48. The whole is missing, so add: □ = 48 + 36 = 84 mangoes.
Worked example 3
A bus left Nyabugogo with 30 passengers. At Kamonyi some got off. At Muhanga, 12 got on. Now there are 35 passengers. How many got off at Kamonyi?
Sentence: 30 − □ + 12 = 35. Undo + 12: 35 − 12 = 23. So 30 − □ = 23, and □ = 7. Check: 30 − 7 + 12 = 35.
Worked example 4
Habimana's avocados: □ − 1 450 + 800 = 2 600.
Step 1 Undo + 800: 2 600 − 800 = 1 800.
Step 2 Undo − 1 450: 1 800 + 1 450 = 3 250.
They had 3 250 avocados in the morning.
Round to check quickly: about 3 300 − 1 500 + 800 = 2 600. That matches the story, so 3 250 is sensible.
Tip
Story, sentence, solve, check. Turn the story into a number sentence with a box. Solve it. Then put your answer back into the story to see if it is true.
Work each one in your exercise book first. Then tap Show answer to check.
A school has 634 pupils. 289 of them are girls. How many are boys?
Answer: 345 boys. 289 + □ = 634, so □ = 634 − 289 = 345.
Ishimwe had some sweets. He gave away 15 and has 27 left. How many did he have?
Answer: 42 sweets. □ − 15 = 27, so □ = 27 + 15 = 42.
A tank had 900 L of water. Some was used, and 350 L is left. How much was used?
Answer: 550 L. 900 − □ = 350, so □ = 900 − 350 = 550.
Uwase had 2 500 Frw. She bought a book, then earned 1 000 Frw. Now she has 2 800 Frw. How much was the book?
Answer: 700 Frw. 2 500 − □ + 1 000 = 2 800. Undo + 1 000: 1 800. Then 2 500 − 1 800 = 700.
"I think of a number. I add 45 and get 100." What is my number?
Answer: 55. □ + 45 = 100, so □ = 100 − 45 = 55.
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
Quick check
Gatete had some marbles. He lost 14 and has 30 left. Which number sentence fits the story?
