A number plus 8 is 21. Find the number.

Step by step

The question

Question: A number plus 8 is 21.

Find the number.

1 Find what we know

What do we know?

A number plus 8 is 21.

What do we want?

The number.

2 Name the unknown

We don't know the number.

So we call it x.

3 Write the equation

Change the words to math, one part at a time.

So the equation is x + 8 = 21.

Now we solve it.

4 Remove the + 8

We want x by itself.

The + 8 is in the way.

To remove + 8, we take away 8.

We do it on both sides, so they stay equal.

+ 8 and − 8 make zero.

Cross them out.

21 − 8 = 13.

So x = 13!

5 Answer the question

The number is 13.

6 Check

Let's check in the story.

Put 13 in the story: 13 plus 8.

It makes 21, like the story says.

So the answer is right!

Let's look back.

First: find what we know.

Then: name the unknown.

Then: write the equation.

Then: solve it.

Then: answer the question.

Last: check.