 So another way of doing subtraction is known as counting past, and again this is based on the idea that we don't have yet any formal algorithm for performing a subtraction, but we know what subtraction means, and we know how to count. So for example, let's take the problem 593 minus 47. And counting past is based on the idea that you can take more than what you're supposed to, but you should return the excess. And we're all familiar with this as adults because this is how we operate with our taxes. We pay out taxes during the year, and then file for a refund because we paid out too much. So let's consider this. We want to find a convenient benchmark number, something that's easy to work with. And so here I'm subtracting 47, so a convenient benchmark number that's a little bit more than 47. Well, it's going to be 50, for example, that's 3 more than 47. So I'll start by subtracting 50 because that's easy to work with. 593 minus 50 is 543, but oh wait, I've gone too far. And so now I have to return 3 back, and I'm going to get 546. Now we don't have to write it out this way. We could also show this process using an arrow diagram. So here I am, 593 subtracted 50. That's the easiest subtraction. Too far, add 3 back to get to 546 as my difference. Well, we can do things that are more complicated this way. And just as a quick note, prior to the modern day, this used to be known, this method is actually very familiar to anybody who's ever had to work a cash register. This is used to, at one point, be known as the method of making change. And the idea is that, well, if you want to make change, then what you could do is you could give too much, and then figure out how much you had to return. So I'm going to subtract 1,987, so probably the nearest benchmark number looks like it's going to be 2,000, and I can either figure that out in my head. I know that's plus 3, plus 10 will take me to 2,000 plus 13, or some variation thereof. And so I'll start by taking too much. If I take 2,000, if I take 2,000, I'll have to give back 10 and 3. So the advantage here is that it's much easier to subtract 2,000. And then I'll give back 3 and 10, and my difference, 4,012 minus 1,997, is going to be 2,025. And again, I can show this using an arrow diagram. So here's my 4,012, take away too much, and get back the 3 and the 10 that we took as our excess.