f(x) = 4x, find x when f(x) = 20

Step by step

The question

Question: find x when f(x) = 20 for f(x) = 4x.

f(x) = 20 means: the answer that comes out is 20.

We look for the x that goes in.

Careful!

It is not f(20).

20 is what comes out, not what goes in.

1 Write the equation

f(x) is 4x.

So 4x = 20.

Now we solve it.

2 Remove the 4

4x means 4 times x.

We want just x.

To remove times 4, we divide by 4.

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.

4 divided by 4 is 1.

So just x is left.

20 divided by 4 is 5.

So x = 5!

3 Check

Let's check: put 5 into f.

We get 20.

So f(5) = 20.

x = 5 is right!

Let's look back.

First: write the equation.

Then: remove the 4.

Last: check.