Given the equation
4r=124
You have to clear the value of r, this is, that r ends up alone in one side of the equation and the rest of the terms of the equation stay in the other side.
As you can see r is being multiplied by 4, to nullify this multiplication you have to "reverse the operation" that is, divide it by four.
And for the equality to continue, every operation made in one side of the equation has to be done in the other side, this means that if you divide 4r by 4, you have to divide 12
