Multiplication (II)
A 2-digit or 3-digit number times a 2-digit number in column form, multiplying by the units and the tens separately and adding the two products; multiplying by 10, 20, 30 and so on; multiplying cleverly by changing the order or grouping (25 × 4 = 100); estimating products; and word problems.
Key words
| Chinese | English |
|---|---|
| 乘數 | multiplier |
| 被乘數 | multiplicand |
| 積 | product |
| 直式 | column form |
| 估算 | estimate |
The 6 short lessons and the chapter test
Lessons are in Chinese for now.
- 乘整十數和拆開來乘
- 兩位數乘兩位數
- 三位數乘兩位數
- 交換位置和巧妙地乘
- 估算
- 應用題
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1計算。
397 × 50 =
Answer and explanation
答案：397 × 50 = 19850
為甚麼？397 × 50 就是 397 × 5 × 10。先算 397 × 5 = 1985，再乘 10，得 19850。
Question 2用直式計算。
88 × 94 =
Answer and explanation
答案：88 × 94 = 8272
為甚麼？把 94 分成 90 和 4：88 × 4 = 352，88 × 90 = 7920，再相加：352 + 7920 = 8272。
Question 3用直式計算。
955 × 25 =
Answer and explanation
答案：955 × 25 = 23875
為甚麼？把 25 分成 20 和 5：955 × 5 = 4775，955 × 20 = 19100，再相加：4775 + 19100 = 23875。
Question 4計算。（想一想：先乘哪兩個數最快？）
25 × 4 × 75 =
Answer and explanation
答案：25 × 4 × 75 = 7500
為甚麼？先算 25 × 4 = 100，再算 100 × 75 = 7500。三個數相乘，先乘哪兩個數，積都一樣。
Based on learning unit 4N1 of the Education Bureau’s Primary Mathematics Curriculum (2017).