Angles on a line 40, 60, x

Step by step

The question

Question: find x when the angles on a straight line are 40°, 60° and x.

1 Use the angle fact

Angles on a straight line add up to 180°.

A straight line is half a turn.

So we add them all and make it 180: 40 + 60 + x = 180.

2 Group like terms

The plain numbers go together.

Now we have 100 + x = 180.

3 Remove the 100

We want x by itself.

The 100 is in the way.

To remove 100, we take away 100.

We do it on both sides.

Why both sides?

The = sign means both sides are the same amount.

If we change only one side, they are not the same any more.

100 and − 100 make zero.

Cross them out.

180 minus 100 is 80.

So x = 80.

The angle x is 80°.

4 Check

Let's check: put x = 80 back and add the angles.

They add up to 180°.

So x = 80 is right!

Let's look back.

First: use the angle fact.

Then: group like terms.

Then: remove the 100.

Last: check.