Grade 3Math
Mental Math with 2-Digit NumbersSplit by place, or make a friendly number
We add and subtract 2-digit numbers in our head, without writing the column method. We use two ideas: splitting the numbers by place, and making a friendly number.
The narration and the pictures are in Japanese. You can hear the same words that are used in class at a Japanese school.
- Video
- 2:06（8 scenes）
- Problems
- 2 problems
Open this lesson in Japanese（2けたの あん算）
Let's try
Write the math sentence in your notebook first, then open the answer.
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1Calculate 68 + 24 with mental math.
Hint
Split the numbers into the tens place and the ones place.
Answer
It is 92. You can calculate 60 + 20 = 80, 8 + 4 = 12, 80 + 12 = 92.
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2Calculate 71 − 38 with mental math.
Hint
Split the number you subtract into the tens place and the ones place, and subtract them one at a time.
Answer
It is 33. You can calculate 71 − 30 = 41, 41 − 8 = 33.
Words in this lesson
- mental math あん算
- Calculating in your head, without using the column method or an abacus.
- friendly number きりのよい 数
- A number whose ones place is 0, like 10, 20, 30. It is easy to calculate with.
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).
- There is more than one way to do mental math. Whether your child "splits by place" or "makes a friendly number and adjusts afterward," the answer is the same. Please let your child choose the way that is easiest for them to think through.
- Calculating a shopping total or the change together in your head at a store helps make it stick. The written column method is covered in the unit "Addition and Subtraction with 3- and 4-Digit Numbers."