Subtraction Word Problems

1Summary 重點摘要

Ate, left away, taken away = subtraction.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Read twiceA subtraction word problem is also read twice: first to understand the story, then to find the numbers and key words. Example: there are 9 buns on a plate and 4 are eaten; how many are left? After reading we know there were 9 at first and 4 went away, so the numbers are 9 and 4, and the question asks 'how many left'. Why twice? The first reading keeps only the story, and the second confirms which number is the starting total and which is what went away. A common mistake is writing the sum after one hasty reading and reversing the two numbers.
2. Step 2: Circle key wordsKey words for subtraction tell us a part is taken away from the original amount, such as 'eaten', 'gave away', 'used up' and 'flew away'; when they appear, the amount gets smaller, so we subtract. Example: in 'there are 9 buns on a plate and 4 are eaten', the word 'eaten' means 4 buns are taken from the 9. Circling the key words keeps addition and subtraction from being swapped. A common mistake is treating 'eaten' as a word for adding, as if the number would grow; in fact the buns on the plate only grow fewer.
3. Step 3: Write the sumWhen a part is taken from the original amount, write the sum with a minus sign: the starting total goes first and the part taken away goes second, and the order cannot be swapped. Example: 9 - 4 = ?, where 9 is the total at the start and 4 is what went away. Why not swap? Subtraction means 'take a part away from the total'; writing 4 - 9 would mean taking 9 buns from 4, which cannot be done. A common mistake is exactly this reversal, and once the sum is wrong, all the work after it is wasted.
4. Step 4: Work it outSubtraction can be worked out by counting back: start from the bigger number and count back as many steps as the smaller number. Example: for 9 - 4, count back 4 steps from 9: 8, 7, 6, 5, so 9 - 4 = 5. Bend one finger for each step while counting, stop after 4 steps, and the number you stop at is the answer. There are two common mistakes: counting forward instead of back, reaching 10 and 11 from 9, or counting the starting number as the first step and going one step too far; both are avoided by counting slowly and checking afterwards.
5. Step 5: Answer in a sentenceThe answer to a subtraction word problem is also written as a full sentence with the unit. Example: when the question asks 'how many buns are left', write 'There are 5 buns left'; the sentence holds the number 5, the unit 'buns', and answers the question's 'how many left'. A bare number without words is not a full answer, and a wrong unit will not do either, for example writing 'birds' for buns. Before writing, read the question once more and answer exactly what it asks; this habit keeps the answer on target.
6. Step 6: CheckSubtraction is checked by adding back: add what is left to what went away and see whether the starting total comes out. Example: for 9 - 4 = 5, work out 5 + 4 = 9; this matches the 9 at the start, so 9 - 4 = 5 is correct. Why does this work? Subtraction and addition are opposite operations: the total minus a part equals what is left, and what is left plus what went away returns to the total. A common mistake is doing the same subtraction again, which is only a repeat and finds nothing; a real check must use addition.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• 'Eaten', 'away', 'gave' mean subtract.
• Write the subtraction: 9 - 4 = 5.
• Count back to subtract.
• Answers need units.
• Check subtraction with addition.