A short instructional transcript of a classroom-style YouTube video (presenter Clare) demonstrating how to use estimation—via rounding—to check the reasonableness of arithmetic calculations. Examples include rounding for addition (3457+5089), subtraction (3742−2177) and multiplication (362×23); the speaker notes that multiplication estimates are typically less precise. The goal is to help students detect and correct large errors quickly.
Chapters
Hello, I'm Clare and today we're going to look at how we can use estimation to help us check our calculations. Ok, let's have a look at what we've got on the board here. We have 3457 and 5089 and the child has written 456. Now, as a teacher that drives you mad. You know, they're way out, but the child hasn't even noticed it. So let's use estimation to help us see that how far out it really is. Okay, I've got 3457. Well, I can round that up to 3500, add 5089. That could be rounded up to 5100. Let's have a look at my estimation now. 3500, add 5100, gives me 8600. I would now get the child to do the calculation again and hopefully they will be much closer this time and see that they've now got the answer right. Let's have a look at a different kind of calculation now. Okay, over here we have a subtraction. I've got 3,742, take away 2,177. Well, I can round my first number down to 3,700, take away 2002, 200. Let's have a look at the estimation. 3,700, take away 2,200 will give me 1,500. And when I do the actual subtraction, I can check and see that actually I'm very close to my estimation, so my answer is probably correct. Okay, now we're going to look at some multiplication. When you do this, you need to warn children that when you're estimating with multiplication, your estimations probably aren't going to be quite as close as they will be when you're adding and subtracting, because obviously when you start to multiply, numbers get bigger much more quickly. Okay, let's have a look at this. I've got 362 multiplied by 23. So 362 can be rounded up to 400. And my 23, I would round that down to 20. Let's have a look at my estimation. I've got 400 multiplied by 20, giving me 8,000. I then do the actual multiplication and I can see that that probably is the right answer. It's as close to 8,000 as I'm probably going to get. So that's how we can use estimation to help us check that our calculations are accurate.
