Grade 3Math
Math Sentences with □Make the unknown number a □, and write the math sentence in the order of the story
Even in a story with a number you don't know, if you make that number a □, you can write the math sentence in the same order as the story. Let's try putting numbers in the □ to check, and finding the □ by looking at a diagram. We try it with addition, subtraction, multiplication, and division stories.
The narration and the pictures are in Japanese. You can hear the same words that are used in class at a Japanese school.
- Video
- 2:26（9 scenes）
- Problems
- 3 problems
Open this lesson in Japanese（□を つかった 式）
Let's try
Write the math sentence in your notebook first, then open the answer.
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1You had some yen. After you bought a notebook for 80 yen, 45 yen was left. Write a math sentence with □, and find how much money you had at first.
Hint
Make the money at first a □, and write the math sentence in the order of the story. In a tape diagram, □ is the money you used and the money left put together.
Answer
The math sentence is □ − 80 = 45. □ = 45 + 80 = 125, so it is 125 yen. Check: 125 − 80 = 45
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2Find the number that fits in the □ in □ × 8 = 56.
Hint
In the 8 times table, look for the fact whose answer is 56.
Answer
□ = 56 ÷ 8 = 7. Check: 7 × 8 = 56
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3In a class of 30 people, some were absent, so 27 people came to school. Make the number of people who were absent a □, write a math sentence, and find □.
Hint
If you subtract the number of people who were absent from the number of people in the class, you get the number of people who came.
Answer
The math sentence is 30 − □ = 27. □ = 30 − 27 = 3, so it is 3 people.
Words in this lesson
- box □
- A sign written in a math sentence in place of a number you don't know.
- putting in 当てはめる
- Putting a number in the □ to check whether the math sentence is right.
- tape diagram テープ図
- A diagram that shows the size of numbers with the length of tapes. You can see how the whole and the parts are related.
For parents and guardians
- The names of lessons and the order of topics differ slightly between textbooks. This page follows the content of Japan's national curriculum guidelines (the Course of Study). Using a tape diagram to decide whether to add or subtract is covered in the 2nd grade unit "Addition and Subtraction with Tape Diagrams."
- The good thing about using □ is that even when there is an unknown number, you can write the math sentence in the order of the story. Sometimes the form of the math sentence and the way to calculate differ, such as finding the answer by subtraction even though the story says "got more." Going in 2 steps, first writing the math sentence in the order of the story and then looking at the diagram to think about how to find the answer, makes it less confusing.
- Putting numbers in the □ to check also works as a way to check the answer. Try building the habit of putting the number found into the □ and saying out loud whether it matches the story. The □ leads to math sentences with letters (such as x) learned in 6th grade.