Four arithmetic operations (II)
Mixed calculations with all four operations: multiplication and division before addition and subtraction, and brackets first; calculations with up to five numbers; using the distributive law to calculate cleverly (38 × 8 + 38 × 42 = 38 × 50); finding the value of one first; estimation and word problems.
Key words
| Chinese | English |
|---|---|
| 四則運算 | four arithmetic operations |
| 括號 | brackets |
| 分配律 | distributive law |
| 運算次序 | order of operations |
The 8 short lessons and the chapter test
Lessons are in Chinese for now.
- 分配律
- 先乘除後加減
- 乘除混合
- 運算次序
- 括號
- 五個數的計算
- 單一數量
- 估算和應用題
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1用分配律計算。
39 × 59 − 39 × 9
= 39 × ( − )
= 39 ×
=
Answer and explanation
答案：39 × 59 − 39 × 9
= 39 × (59 − 9)
= 39 × 50
= 1950
為甚麼？兩個乘式都有 39，可以先把 59 和 9 相減，得 50，再乘 39：39 × 50 = 1950。
Question 2把 99 看成 100 − 1，用分配律計算。
58 × 99
= 58 × 100 − 58 × 1
= −
=
Answer and explanation
答案：58 × 99
= 58 × 100 − 58 × 1
= 5800 − 58
= 5742
為甚麼？58 × 100 = 5800，58 × 1 = 58，5800 − 58 = 5742。
Question 3計算。
35 − 33 ÷ 3 =
Answer and explanation
答案：35 − 33 ÷ 3 = 24
為甚麼？先乘除後加減：先算 33 ÷ 3 = 11，再算 35 − 11 = 24。
Question 4計算。
2 × 72 ÷ 3 × 6 + 94 =
Answer and explanation
答案：2 × 72 ÷ 3 × 6 + 94 = 382
為甚麼？先乘除：2 × 72 = 144，144 ÷ 3 = 48，48 × 6 = 288；後加減：288 + 94 = 382。
Based on learning unit 4N5 of the Education Bureau’s Primary Mathematics Curriculum (2017).