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Estimating with multiplication

Sal estimates to find reasonable products to 1-digit by 2, 3, and 4-digit multiplication expressions.

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Video transcript

- [Instructor] In this video, we're gonna get a little bit of practice estimating with multiplication. So over here it says question mark is, and then you have this squiggly equal sign. And so you can view that squiggly equal sign as being what does this roughly equal to? It doesn't have to be exactly right. So what is roughly equal to 58 times six? And so which of these choices would you pick? Pause the video and see if you can answer that. So once again, we don't have to get this exactly we just need to estimate what 58 times six is. And the way that I would tackle it is hey, can I rewrite 58 as a number that is easy to multiply by six or easy to multiply in general? And the easiest thing I can think of is 58 is awfully close to 60. So if you say that that is close to 60 if we're just estimating it, well what would 60 times six be? Well 60 times six is the same thing as, actually let me just write it this way. You can view 60 times six as six times 10 times six, which you could also view as six times six times 10. And what's six times six? That's 36. And then 36 times 10 is 360. So I would go with that choice right over there. Notice, these other choices aren't anywhere close to that so we feel pretty good about our estimation. Let's do another example. So here we're asked what's three times 2,890? What's that roughly equal to? How would you estimate this? Pause the video and see if you can figure that out. Alright, so 2,890 is not an easy thing to maybe multiply in our heads, but I can say hey, that's kinda close to 3,000 so I'm gonna say that this is going to be approximately equal to, or roughly equal to, three times 3,000. Now what's three times three? That's nine. So what's three times 3,000? We'll it's gonna be nine thousands. So it would be this choice right over here. If what I just did was a little bit confusing, you can view three times 3,000 as three times three times 1,000. This part right over there is 3,000. Three times three is the nine, and you have nine thousands, 9,000. Let's do one more example. This is going to involve fairly large numbers. So what is, if you had to estimate 5,111 times nine. Which of these would you pick? Pause the video and answer that. Alright, so I would view this, this is roughly, I would estimate it, as 5,000 times 9. And so 5,000 times nine, that's the same thing as five times 1,000 times nine, which is the same thing as five times nine times 1,000. Five times nine is 45, so it's gonna be 45 thousands, or 45,000 which is this choice right over there.