This lesson builds upon the concepts we've covered before,

that is, resistors in series where I can replace

a series set of resistors with one resistor with this formula.

Or resistors in parallel where the formula for

the equivalent resistance is right here.

Let's look at this example.

The way to approach something like this is to first recognize resistors in series or

resistors in parallel and to replace them with their equivalent resistance.

Now, people looking at something like this.

A common mistake that they make is to think that these two resistors are in

series with one another, they're not.

In order to be in series, you have to have the same current flow through them.

But the problem is the current flowing this way, part of it could go this way and

part of it this way.

So it's not the same current.

So they are not in series with one another.

However, these two are in series with one another

because the same current has to flow through both of them.

So I can take these two.

And I'm going to redraw the circuit with that simplification.

And that's what I usually do on here.

I draw a sequence of circuits.

Each time I simplify, I redraw the circuit.

So that's 20.

And this is a sum of the 2 is 20.

And this is 30.

So now it's much clearer because I've redrawn it that these two are in

parallel with one another.

So then if I redraw that one I

used two resistors that are in parallel in fact, these are the same and

whenever they are the same, the equivalent is going to be half of that.

And then that's 30.

And now I've got two resistors that are in series with one another.