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Lesson 15: Word Problems Involving Subtraction
In this lesson you'll learn how to identity word problems that require subtraction, and you'll learn how to solve them.
Don't forget to watch the embedded video clip for this lesson at the bottom of the page. Please be sure to read the embedded video information and disclaimer.12 apples, and he gives away 5 of them. How many apples does he have left?
Here you can see that I've highlighted the words "left" and "gives away," but this will not be done for you on a test. To solve a word problem, first read the problem carefully to look for words that tell you what mathematical operation to do. When we see words like the ones that are highlighted, and no other special keywords, we know that we'll be subtracting. The answer to the problem is 7. You just subtract the apples.
Always look carefully to see if there are any other keywords in the problem. If there are, it's possible that you will have to first do some other operation, such as multiplication, and then figure out the total number of items that you have. These problems sometimes have a multiple choice answer that tries to trick you if you weren't careful to find all of the keywords in the problem, and if you just assumed that you were subtracting.
When you subtract, the answer that you get is called the difference. If a word problem ever asks you find the difference of two numbers, they are just testing to see if you know that you are supposed to subtract. Certainly if a problem says that there are a number of items minus another number of items, it means that you should subtract them.
Sometimes there is no obvious keyword that tells you to subtract. For example, a problem might read: Joey has 12 apples and Mary has 4 apples. How many more apples does Joey have than Mary? In this case there is no specific keyword, and when you see the word "more," you might be tempted to just add. However, what the problem really wants to know is the difference between the number of apples that the children have. We have to subtract 12 - 4 to find the answer.
Always read a word problem carefully and slowly, determine what operation you are being asked to do, and then convert the problem into standard notation.