Multiplication (I)
A 2-digit or 3-digit number times a 1-digit number, with carrying, in column form and why it works (43 × 2 is 40 × 2 plus 3 × 2); multiplying three numbers, and seeing that the order of multiplying does not change the answer, which can make it quicker; estimating products; and word problems.
Key words
| Chinese | English |
|---|---|
| 乘法 | multiplication |
| 積 | product |
| 直式 | column form |
| 進位 | carry |
| 估算 | estimate |
The 6 short lessons and the chapter test
Lessons are in Chinese for now.
- 拆開來乘
- 進位
- 三位數乘一位數
- 三個數相乘
- 估算
- 應用題和重溫
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1把 47 拆成 40 和 7，再計算。
47 × 4 = 40 × 4 + 7 × 4
= +
=
Answer and explanation
答案：47 × 4 = 40 × 4 + 7 × 4
= 160 + 28
= 188
為甚麼？47 × 4 就是 40 × 4 和 7 × 4 的和：160 + 28 = 188。
Question 2用直式計算。
54 × 8 =
Answer and explanation
答案：54 × 8 = 432
為甚麼？個位：4 × 8 = 32，寫 2 進 3；十位：5 × 8 + 3 = 43。所以 54 × 8 = 432。
Question 3用直式計算。
854 × 7 =
Answer and explanation
答案：854 × 7 = 5978
為甚麼？個位：4 × 7 = 28，寫 8 進 2；十位：5 × 7 + 2 = 37，寫 7 進 3；百位：8 × 7 + 3 = 59。所以 854 × 7 = 5978。
Question 4計算。
3 × 2 × 9 =
Answer and explanation
答案：3 × 2 × 9 = 54
為甚麼？由左至右：先算 3 × 2 = 6，再算 6 × 9 = 54。
Based on learning unit 3N2 of the Education Bureau’s Primary Mathematics Curriculum (2017).