Estimating to subtract multi-digit numbers | Grade 5 (TX TEKS) | Khan Academy

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 761,023 minus 18,965 is. 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.

All right, now let's work through it together. I always enjoy estimating because, frankly, it requires less work. You're able to do things very roughly.

So here I have two numbers that go all the way to the 10,000's place. So I'm just going to 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 10,000 place while this number goes to the hundreds of thousands. To make it consistent, I'm going to round both to the 10,000's place because if I just rounded this one to the 100,000s, it'll just become zero, and I wouldn't be subtracting anything.

So if I round them both to the 10,000's 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 in 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.