Addition and subtraction (II)
Adding 3-digit numbers with carrying, up to three numbers at a time; changing the order of numbers to make a hundred first (the commutative and associative properties); subtracting 2-digit numbers with borrowing; working from left to right when taking away two numbers; estimating answers; and word problems.
Key words
| Chinese | English |
|---|---|
| 進位 | carry |
| 退位 | borrow (regroup) |
| 直式 | column form |
| 由左至右 | from left to right |
| 估算 | estimate (an answer) |
The 7 short lessons and the chapter test
Lessons are in Chinese for now.
- 三位數加法
- 三個數相加
- 退位減法
- 三個數相減
- 應用題
- 估算
- 重溫
- 章節測驗 (chapter test)
Sample questions (in Chinese)
Question 1計算。
47 + 395 =
Answer and explanation
答案：47 + 395 = 442
為甚麼？個位：7 + 5 = 12，寫 2，向十位進 1；十位：4 + 9 + 1 = 14，寫 4，向百位進 1；百位：0 + 3 + 1 = 4。所以 47 + 395 = 442。
Question 2用直式計算。
346 + 71 =
Answer and explanation
答案：346 + 71 = 417
為甚麼？直式要對位，由個位開始加。個位：6 + 1 = 7；十位：4 + 7 = 11，寫 1，向百位進 1；百位：3 + 0 + 1 = 4。所以 346 + 71 = 417。
Question 3計算。
64 − 48 =
Answer and explanation
答案：64 − 48 = 16
為甚麼？個位：4 不夠減 8，向十位借 1，14 − 8 = 6；十位：6 借了 1 給個位，剩 5；5 − 4 = 1。所以 64 − 48 = 16。
Question 4由左至右計算。
52 − 13 − 9 =
Answer and explanation
答案：52 − 13 − 9 = 30
為甚麼？三個數相減，要由左至右計算：52 − 13 = 39，39 − 9 = 30。
Based on learning unit 2N2 of the Education Bureau’s Primary Mathematics Curriculum (2017).