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Course: Grade 5 (TX TEKS) > Unit 1
Lesson 1: Estimate addition and subtraction of whole numbers- Estimating multi-digit sums
- Estimate to add multi-digit numbers
- Estimating to subtract multi-digit numbers
- Estimate to subtract multi-digit numbers
- Estimating multi-digit addition and subtraction word problems
- Multi-digit addition and subtraction estimation word problems
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Estimating multi-digit sums
Use estimation to add multi-digit numbers. Created by Sal Khan.
Video transcript
- [Instructor] Let's get a little bit of practice estimating
adding large numbers. So if someone were to walk
up to you on the street and say quickly, roughly what is 493,792 plus 205,218, what does that roughly equal to? And sometimes people will put this little squiggly equal sign, that means approximately equal to what? And so you have to figure
this out in your head. You can't get a piece of paper out and try to add place by place. So there's a bunch of ways
that you could do this, but the way that I would
do that is that both of these are in the hundreds of thousands. So in my head, I would round them to the closest 100,000 and then add them up in my head. So what I'm about to show
you, I'm gonna write it down, but this is what I would
be doing in my head. So 493,792, if I were to round
that to the nearest 100,000, that's approximately equal to 500,000 and 205,218, if I were to round that to the nearest 100,000, that's approximately equal to 200,000. And now it's pretty easy to do in my head, 500,000 plus 200,000,
well, that's 700,000. So this whole thing is
approximately equal to 700,000. Now, let's give another
example where the two numbers, one's in the thousands and
one's in the tens of thousands. So let's say we have 64,895 plus 3,117. And once again, I just wanna estimate what this sum is equal to. I don't need to get the exact answer. How would you approach this? So it might be tempting to round this number
to the nearest 10,000, which would get you 60,000 and then round to this number,
to the nearest thousand, which would get you 3,000 and then add them
together, you'd get 63,000. But something feels a little bit off there because you wouldn't
expect 64,895 plus 3,117 to be less than 64,895. And the reason why we're
getting that strange estimation is because if we're rounding
one to the nearest thousand, we should round the other
to the nearest thousand. So let's not do it this
way, let's round them both to the nearest thousand. So 64,895 to the nearest thousand would be 65,000. And then in my head, I would say, okay, 65,000 plus 3,000 is going to be 68,000. So this is going to be
approximately equal to 68,000.