Decimals (III)
Multiplying by 10, 100 and 1000 moves the decimal point one, two or three places to the right, and multiplying by 0.1, 0.01 and 0.001 moves it to the left; a decimal times a whole number and a decimal times a decimal (one or two decimal places), placing the decimal point in the product; rounding products to tenths or hundredths and writing ≈; estimation and word problems.
Key words
| Chinese | English |
|---|---|
| 小數乘法 | multiplying decimals |
| 小數點 | decimal point |
| 近似值 | approximate value |
| 大約等於（≈） | approximately equal to (≈) |
The 7 short lessons and the chapter test
Lessons are in Chinese for now.
- 乘以 10、100 和 1000
- 乘以 0.1、0.01 和 0.001
- 小數乘整數
- 小數乘小數
- 積的近似值
- 估算
- 應用題
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1計算。
44.1 × 100 =
Answer and explanation
答案：44.1 × 100 = 4410
為甚麼？乘以 100，小數點向右移兩個位；位數不夠，要在末尾補 0：44.1 × 100 = 4410。
Question 2計算。
90.61 × 0.001 =
Answer and explanation
答案：90.61 × 0.001 = 0.09061
為甚麼？乘以 0.001，小數點向左移三個位；位數不夠，要在前面補 0：90.61 × 0.001 = 0.09061。
Question 3用直式計算。
40 × 7.13 =
Answer and explanation
答案：40 × 7.13 = 285.2
為甚麼？先不理小數點，計算 713 × 40 = 28520。因數有 2 位小數，所以積也有 2 位小數：285.20，末尾的 0 可以刪去，即是 285.2。
Question 4計算。
0.68 × 3.17 =
Answer and explanation
答案：0.68 × 3.17 = 2.1556
為甚麼？先不理小數點，計算 68 × 317 = 21556。兩個因數共有 2 + 2 = 4 位小數，所以積也有 4 位小數：2.1556。
Based on learning unit 5N4 of the Education Bureau’s Primary Mathematics Curriculum (2017).