Estimating to subtract multi-digit numbers
The narrator repeatedly says '10 thousandths place' when he means the ten-thousands place. The interventions use the correct term, 'nearest 10,000'.
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This video estimates differences of multi-digit numbers by rounding. It rounds 51,384 and 28,251 to the nearest 10,000 to get about 20,000, and rounds 761,423 and 18,965 to the nearest 10,000 to get about 740,000, explaining why both numbers should be rounded to the same place.
You'll be able to
- round multi-digit numbers to the nearest 10,000.
- estimate a difference by rounding both numbers first.
- explain why rounding both numbers to the same place gives a sensible estimate.
Key words
- estimate
- A rough answer that is close to the exact one and quick to work out.
- approximately equal to
- The squiggly equal sign, meaning 'roughly the same as'.
- round to the nearest 10,000
- Change a number to whichever multiple of 10,000 it is closest to.
Transcript
- [Presenter] So we have two subtraction problems here that I want you to estimate. I first want you to estimate what 51,384 minus 28,251 is, and then I want you to estimate what 761,423 minus 18,965 is. And this little squiggly equal sign means approximately.
So you're only creating an estimate. You don't have to do all of the detailed math here. Pause this video and see if you can figure those out. Alright, now let's work through it together. I always enjoy estimating because it frankly requires less work. You're able to do things very roughly.
So here, I have two numbers that go all the way to the 10 thousandths place. So I'm just gonna round them both to the nearest 10,000. So 51,384 rounded to the nearest 10,000 is 50,000 and 28,251 rounded to the nearest 10,000 is roughly 30,000. So in my head, I would say 50,000 minus 30,000, well, that's going to be 20,000.
So this whole thing is approximately equal to 20,000. Now let's look at this scenario right over here. This number goes up to the 10 thousands place. Well, this number goes to the hundreds thousands. So to make it consistent, I'm going to round both to the 10 thousandths place.
'Cause if I just rounded this one to the a hundred thousands, it'll just become zero and I wouldn't be subtracting anything. So if I round them both to the 10 thousandths place, then this one is going to be roughly equal to 760,000. And then this number is going to be roughly equal to 20,000.
If I round to the nearest 10,000 and on my head, if I said, okay, 760,000 minus 20,000, I could do that in my head, that's going to be 740,000. So the difference of these two numbers is approximately 740,000, and I'm done.
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