Estimating multi-digit addition and subtraction word problems
The river question asks 'about how many meters longer' but both lengths and the narrator's answer are in kilometers. A non-pausing note flags the unit mix-up.
The check-ins use the correct version. The video is Khan Academy's, unchanged.About this video
This video shows how to estimate the answers to addition and subtraction word problems with big numbers. You round each number to the same place value, such as the nearest ten thousand or nearest thousand, and then add or subtract the rounded numbers.
You'll be able to
- round large numbers to the nearest thousand or ten thousand.
- estimate a sum or difference by rounding both numbers to the same place first.
- tell when a word problem wants an estimate instead of an exact answer.
Key words
- estimate
- A close, reasonable answer that does not have to be exact.
- round
- To change a number to a nearby, simpler number, like the nearest thousand.
- nearest 10,000
- The multiple of 10,000 that a number is closest to, such as 160,000 for 158,718.
Transcript
- [Instructor] We're told Minli has 158,718 flight points, I guess those are like frequent flyer miles, through an airline award program. She gained 21,118 flight points. About how many total flight points does Minli have now? So why don't you pause this video and have a go at it.
And remember, they don't want you to figure out the exact number. They just say about how many, so you just need to have a reasonable estimate. Alright, now let's work through this together. So 21,118 flight points, if I were to round it to the nearest 10,000, that's going to be about 20,000 flight points.
And then 158,718, if I round that similarly to the nearest 10,000, I wouldn't wanna round it to the nearest hundred thousand because then I'm rounding it at different levels. So if I round it to the nearest 10,000, then that is going to be approximately 160,000 flight points.
And then if I were to add those two numbers together, that means that Minli has about 180,000 frequent flyer miles or flight points. Let's do another example. So here, they're telling us the Nile River is 6,650 kilometers. The Tigris River is 1,950 kilometers, I'm assuming kilometers long.
About how many meters longer is the Nile than the Tigris River? So pause this video and have a go at that. Alright, let's work through this. So let's see, they both are in the thousands, so let's just round them both to the nearest thousand. The Nile River, if I were to round it to the nearest thousand, is approximately 7,000 kilometers.
And the Tigris River, if I round it to the nearest thousand, is approximately 2000 kilometers. So if I were to estimate this, I would say, how much longer is the Nile than the Tigris? It would be 7,000 - 2000, which is approximately, well, that's 5,000, but I'm estimating the difference between these two numbers, which would be approximately 5,000 kilometers.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.