Estimating by Rounding - Lesson 36

This is the video for lesson 36 on my web site, Estimating by Rounding. Before watching this video, you may want to watch the video for lesson 12 where I showed you the actual procedure for rounding numbers. This lesson just discusses estimating by rounding in a general sense. Sometimes we’re given a problem that will ask us to estimate an answer. And granted this is very vague and the instructions are very vague. What the problem is asking us to do is just use our common sense to calculate an approximate answer. Let me show you an example. Let’s say you’re told to estimate 307 plus 991. And again, we’re not being asked to find actual answer. We’re being asked to estimate. There is no real right or wrong way to do this. What you do is you look at each number and decide the most logical way of rounding. Now when I see 307, what I say to myself is what probably makes the most sense in this problem is to just round it to the nearest 100, which is 300. A person could argue, “Well, why don’t we make it 310 since it is closer to 310 than it is to300?” But in the case of this problem, it should be sufficient to just make it 300. Now in the case of 991, again in this case, someone would be justifying it and saying let’s make it 990 but I see that’s it very close to a 1000 so I could actually make it a 1000 just to make my job a little easier. And of course 300 plus a 1000 is 1300. Now, I can use this symbol, which means approximately. I can say that 307 plus 991 is approximately 1300. We got an estimated answer by rounding. One time when it's very important to do something like this is whenever you’re asked to get the answer to any question, you should always do something like this in your head. It's important before doing a problem just to get a rough idea of what you think the answer will be. In this way if you make a big mistake and you get any answer to that, that’s totally, totally different, you’ll know it's definitely wrong. If you got an answer that is close to it, it maybe hard to know if you are right or wrong. But if you’re answer is totally off, you’ll know that it's wrong and you know to go back and check your work so it is a good idea to always get into the habit of estimating your answers. In this case we’re estimating our answers by rounding. There's other ways that will learn of estimating. Let’s do another example, 759 plus 22 plus 3891. Again, there is no real right or wrong way to do this. You have to look at the problem and just see what kind of make sense as far the best way to round. Now in the case of this first number, it's kind of roughly halfway between 700 and 800 so I'm not going to round it to the nearest 100. What I think makes sense in this case is to round to the nearest 10 and so I’ll make it 760. Now in the case of the 22, it's between 20 and 30. I'm going to go ahead and make it 20. That will make our arithmetic easy and it's pretty close to that. Now in the case of this number, we could choose to round to the nearest 100 which would make it 3900. But just looking at it, I'm going to go ahead and actually round to the nearest 1000 because it's it not that far from 4000. So to make my job easier for adding later, I'm just going to round this up to 4000. Remember, we’re just getting an estimated answer. And then adding all this up, we can see that answer is 4780. So, we can say that this sum is approximately equal to 4780. It’s not exactly equal to that but it is close. It gives us a rough idea of what we expect our sum to be. Okay this is just a basic lesson on rounding. Get into the habit of always doing something like this for any problem. Just take an extra few seconds whether it's for your homework or on a test. Take a few moments to estimate what you think the answer should be. Just get a rough idea. That will help you avoid many care—.