Word problems are short stories that describe a real-life situation using numbers. To solve them, you must figure out whether to add or subtract to find the answer.
A bookstore had 315 books. A school ordered 148 more books. How many books does the bookstore have now?
Think: The store is getting more books. When we combine amounts or find a total, we add.
Look for clue words like "more," "total," "altogether," or "in all" for addition. Draw a picture if it helps you understand the story.
Subtraction is used when you need to find how much is left, how many more are needed, or the difference between two amounts.
A factory made 729 toy cars. They shipped 265 of them to a store. How many toy cars are left in the factory?
Think: Some cars were taken away. When something is removed or we find what is left, we subtract.
Look for clue words like "left," "remain," "difference," "how many more," or "how many fewer" for subtraction.
Some problems require two steps to solve. You must find an answer in the first step and then use it to find the final answer.
Emma collected 127 seashells. Ben collected 84. They put them all in a jar, but then gave 55 shells to their friend. How many shells did they keep?
Step 1: Add to find the total shells collected.
Step 2: Subtract the shells they gave away.
They kept 156 shells.
Read the entire problem first. Identify what you need to find out first before you can get the final answer. Label your steps "Step 1" and "Step 2."
Some problems do not ask for a total or a remainder. Instead, they give you the total and one part, and you must find the other missing part.
There are 400 students in the third grade. If 178 are boys, how many are girls?
Think: You know the total (400) and one part (178 boys). To find the other part (girls), subtract the part you know from the total.
There are 222 girls.
When a problem gives you a whole and one part, the operation is always subtraction: Whole – Part you know = Other Part.
It is important to check if your answer makes sense. You can check an addition problem with subtraction, and a subtraction problem with addition.
You solved: 542 – 308 = 234
Check with Addition: If you took away 308 and got 234, then adding 234 and 308 should give you the original number, 542.
The check works, so the answer is correct.
Always ask yourself: "Is my answer reasonable?" If the problem is about people, your answer should not be a fraction. Reread the problem with your answer in it to see if it makes sense.