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Current time:0:00Total duration:3:08

CCSS.Math:

we are told that a racecar driver has 28 cars each car has four tires he has to replace all the tires on the cars he has 22 tires right now estimate the total number of tires he needs to buy so pause this video and see if you can do that and the key word here is estimates you don't have to figure out exactly you seem to figure it out roughly the total number of tires he needs to buy all right now let's work through this together and as I mentioned the key word here is estimate and estimating is something we'll do throughout our lives I probably do it once a day at least and when you're estimating you want to make all the numbers roughly right but also think about hey what what I need a how do I need to think about them in order to make the computation a little bit easier or a little bit friendlier so if I wanted to figure out exactly how many tires he needed to buy I would say hey he has 28 cars he has 28 cars each car has four tires so each car has four tires so if you wanted to know the total number of tires you would multiply 28 times four and then it says he has to replace all the tires on the car and if he didn't have any this is the number that he would have to replace but it says he has 22 tires right now so if we were trying to figure out exactly you would say 28 times four that's how many he needs to replace but he already has 22 so you subtract that out and then this whole number would be the number he needs to buy now this calculation isn't so easy to do especially if you're doing it on a napkin or in your head but that's why we're going to estimate so if we're estimating we say hey what's up what's a friendlier number than 28 well 28 pretty close to 30 so I could say so if I say hey 28 is pretty close to 30 so that's our way of making the 28 a little bit friendlier 4 is already pretty easy to deal with and then the 22 well that could be a lot easier to deal with if instead of it being a 22 if that were let's say that's approximately equal to 20 so 22 is approximately roughly equal that's what these squiggly equal signs mean to 20 and so simplifies the calculation a lot we could say that this thing up here is roughly equal to he roughly has 30 cars and each of those cars need four tires and then he's going to figure out the total number of tires he needs to replace but then he subtracts out the number that he already has so instead of subtracting out 22 we're gonna subtract out roughly how many he already has roughly 20 and this computation right here is a lot easier to do in our head three times four is 12 so 30 times four is 120 so this part right over here is 120 and then you subtract 20 from that well 120 minus 20 well that is going to be 100 so he roughly needs if I were to estimate the total number of tires he needs to buy it would be about 100 tires