Addition and subtraction (III)
Subtracting 3-digit numbers, including borrowing across a zero, such as 406 − 118; checking a subtraction by adding; mixed addition and subtraction, worked from left to right; working out change; estimating answers; and two-step word problems.
Key words
| Chinese | English |
|---|---|
| 被減數 | minuend (the number taken from) |
| 退位 | borrow (regroup) |
| 驗算 | check (an answer) |
| 找贖 | change (money given back) |
| 加減混合 | mixed addition and subtraction |
The 9 short lessons and the chapter test
Lessons are in Chinese for now.
- 三位數減法
- 直式減法
- 連續退位
- 驗算
- 加減混合
- 找贖
- 應用題
- 估算
- 重溫
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1計算。
254 − 125 =
Answer and explanation
答案：254 − 125 = 129
為甚麼？個位：4 不夠減 5，向十位借 1，14 − 5 = 9；十位：5 借了 1 給個位，剩 4；4 − 2 = 2；百位：2 − 1 = 1。所以 254 − 125 = 129。
Question 2用直式計算。
705 − 407 =
Answer and explanation
答案：705 − 407 = 298
為甚麼？十位是 0，要先向百位借。個位：5 不夠減 7，向十位借 1，15 − 7 = 8；十位：0 不夠借，先向百位借 1 當作 10，借 1 給個位後剩 9；9 − 0 = 9；百位：7 借了 1 給十位，剩 6；6 − 4 = 2。所以 705 − 407 = 298。
Question 3由左至右計算。
323 + 386 − 353 =
Answer and explanation
答案：323 + 386 − 353 = 356
為甚麼？加減混合也要由左至右計算：323 + 386 = 709，709 − 353 = 356。
Question 4414 − 251 = 163。用加法驗算，填上空格。
163 + 251 =
Answer and explanation
答案：163 + 251 = 414
為甚麼？把答案加回減去的數：163 + 251 = 414，得回原來的 414，所以答案正確。
Based on learning unit 2N5 of the Education Bureau’s Primary Mathematics Curriculum (2017).