Lesson 10 — Activity 2:

Choose the Operation

The process of "choosing the operation" involves deciding which mathematical operation (addition, subtraction, multiplication, or division) or if a combination of operations will be useful in solving a word problem.

So, why is this important? Well, say you follow the 4-Step Method and do everything correct, and then you choose the wrong operation to answer the question. If you do that, then all of your hard work has been for nothing because you will get the wrong answer! And you definitely don't want to do all that work for nothing. So it is important to think about what the question is asking you, so you can choose the correct operation to get the right answer!

For example, one way to solve the following problem is to think of it as a problem of subtraction.

If there are 18 students, and 6 students are not here today, how many are present?

18 – 6 = ?

The phrase not here signals the concept of taking away — or subtraction.

In comparison, the following problem can be thought of as a problem solved by addition.

If there are twelve students in class today and six students are absent, how many are there in all?

12 + 6 = ?

Alternatively, the phrase in all signals a problem solved by addition.

Here is another example using the 4-Step Method (Note: Steps 2 and 3 are together for this problem — plan and solve):

Choosing the operation can sometimes be difficult when answering word problems. Below are some strategies you can use that will help you.

Identify Key Words

Keywords are also called clue words and offer valuable hints to help solve word problems.

To use keywords:
Read over the problem several times.
Identify and underline the keywords.
Determine their meanings.

Keywords often provide clues about which math operation to use. When you are solving word problems, use the chart below to help you figure out the math operation to use.

Think Aloud

Sometimes thinking aloud helps you to figure out what the question is asking. In the think-aloud strategy, you say out loud what you are thinking about when reading and solving math problems.

For example, consider the following problem.

Julia took 20 dollars to the mall. She bought earrings for 3 dollars and a bracelet for 7 dollars. How much did she have left?

If you were thinking aloud to figure out what to do, you might say this:

First, I added 3 plus 7 dollars because it said "3 dollars and 7 dollars" so I knew that meant to add. So, that was 10 dollars. Then I subtracted 10 dollars from 20 dollars because it said "How much did she have LEFT," so I knew that meant to subtract. Therefore, $20 – $10 = $10, so Julia had $10 left.

Try this!

Try answering the word problem below. Use the think -aloud strategy to help you figure out what the question is asking.

Remember to look at the clue words in the problem to help figure out what operation you need to use to answer the question.

When you think you have the answer, click on the tab below to check it!

Leah bought 18 large marbles and 2 bags of small marbles. There were 7 marbles in each bag. Each bag was 14 centimetres tall. How many marbles did Leah buy in all?

Click here to check your answer!

If you talked through your answer, you might say something like this:

First, I need to find the clue words that will tell me what I need to do to answer the question. I see the words "in all" at the end of the problem, so I know that I need to add. I already know that she bought 18 large marbles, but I don't know how many small marbles she bought, because it says that she bought 2 bags. It says that there are 7 marbles in each bag. I know that 2 x 7 = 14, so that means that she bought 14 small marbles. Now, the problem tells me that each bag was 14 centimetres tall. This is extra information that I don't need, so I'm going to ignore it. Now all I have to do is add my two numbers together. My equation will look like this:

18 marbles + 14 marbles = 32 marbles

Leah bought 32 marbles in all!