Rational number word problem: school report

Joelle is working on a school report in a word processor. She wants to place a figure which is 13.09 centimeters wide so that it appears centered on the page. The width of the page is 21.59 centimeters. How much margin should she leave on the left so that the figure is centered? So let's visualize what's going on. We have a page that's 21.59 centimeters wide. So this distance right over here is 21.59 centimeters. So this right over here, that is our page. Let me draw the actual page. Let me draw that in white, since paper tends to be white. So this is her page. It's 21.59 centimeters wide. It's just like that. It's a little unusual dimension for a piece of paper. But we'll go with it. And then she wants to place a figure that is 13.09 centimeters wide in the center of the page. So the figure-- I'll do it in magenta. That's not magenta. The figure is 13.09 centimeters. So let's say that this is the figure right over here. So this is the figure that she wants to place in the middle of the page. So this is 13.09 centimeters. So the question is, how much margin should she leave on the left so that the figure is centered? So they're really asking, what should this distance be here so that the figure is centered? And the key thing is, if the figure is centered, this distance is going to be the same as this distance, so that the figure is right in the middle. Whatever this is, it's going to be the same as this. So one way we could think about it-- we could say, OK, let's take 13.09 centimeters from 21.59 centimeters. And if we do that, then we'll know how much leftover space this plus this combined is. And then we just want to split that space in half. We want half of it to go on the right side, and we want half of it to go on the left side. So let's see how much this left margin and the right margin combined need to be. And that's just 21.59. That's just going to be 21.59 centimeters minus 13.09 centimeters. And this, we get to-- let's see. This is 8.50 centimeters, which is the combined margins. Not just the left or not just the right-- this is the left plus the right margin, is 8.50 centimeters. This is this distance plus this distance. Now, if we want to figure out what the left margin should be, if we want to center it. Well, we just want to split this evenly between the left and the right margin. So we just want to divide this by 2. So if you divide 18.50 by 2-- or multiply it by 1/2, I guess. That might be a little bit more obvious-- either way. So if we multiply this by 1/2, you are left with 1/2 of 8.50, is 4.25 centimeters. So if we want it centered, this is going to be 4.25 centimeters. And of course, this is going to be 4.25 centimeters, as well. And you see, when you add 4.25 to 4.25, you get 8.5 centimeters. And then you add that to 13.09. You get the entire width of the piece of paper. So how much margin should she leave on the left? 4.25 centimeters.