Shelagh: So the next equation is, power equals energy divided by time.

Alison: Yeah.

Shelagh: And, er, I'd like you to try and make t the subject. Now how have we started on the previous equations?

Alison: Well, we've reversed equation to put the subject on the left-hand side, the subject that we want.

Shelagh: Yeah, but if we reverse this one ...

Alison: Yeah.

Shelagh: ... so we have E over t equals P.

Alison: Well.

Shelagh: Where is the t?

Alison: The t is on the bottom and we need, er, the t to be on the top to make it the subject.

Shelagh: That's right. It's on the left but it's not on the top.

Alison: Yeah.

Shelagh: So, that's a perfectly valid equation but it doesn't get us where we want to go.

Alison: Yeah.

Shelagh: So we need to have a different strategy this time. So let's go back to the original, P equals E over t. So what could you do to get t on the top on the left?

Alison: Well as we're dividing by t on that side, if we multiply both sides of the equation by t that would put, um, t on the top on this side so it would be ...

Shelagh: Yeah, go on.

Alison: So it would be P times t and then E over t times t.

Shelagh: t, yeah.

Alison: And then the two ts on the right hand side will cancel.

Shelagh: Indeed.

Alison: So we've got P t is equal to E.

Shelagh: Great.

Alison: But we've still got t multiplied by P.

Shelagh: Yes, so you know what to do to get rid of the P, don't you.

Alison: Yes. I still need to divide both sides by P. So divide the left-hand side by P.

Shelagh: Yes.

Alison: And then those two Ps cancel and t is equal to E over P and it's now the subject.

Shelagh: Excellent. Brilliant. Well done.

Narrator: Let's look again at the hints that Alison used this time.
Hint 3: If the term you want is multiplied by another expression, divide both sides of the equation by that expression.
Hint 4: If the term you want is divided by another expression, multiply both sides of the equation by that expression.
Hint 7: Don't be afraid of using several small steps to rearrange one equation.
Hint 8: Aim to get the new subject into position on the left-hand side as soon as you can.
It's worth thinking a bit more about hints 3 and 4. These are special cases of a more general rule, namely, that to undo any operation, you should do the opposite. So to undo a multiplication you divide, to undo a division you multiply, to undo a subtraction you add and to undo an addition you subtract.