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Multiplication, division word problem: pedaling

Using multiplication and division to make estimates. Easy? Maybe, but what's more important is that you try. Created by Sal Khan.

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Video transcript

Mark pedals his bike 70 times in a minute. Which of these are reasonable estimates about Mark's peddling? Mark all that applies. So let's just think about this a little bit. 70 times in a minute. So there are 60 seconds in a minute. So if he's doing 70 times in a minute, if he was doing 60 times in a minute, then he would be peddling once per second. Since he's doing 70 times in a minute, he's doing a little bit more than once per second. So maybe that'll help us right over here. So here it says Mark peddles his bike what? 4,000 times every second. Well, that's nuts. If he peddled his bike 4,000 times every second, you'd have a way huger number right over here. You'd have 4,000 times per second times 60 seconds per minute. You would have a huge number right over here. If it's 70 times in a minute, we just reasoned that he peddles a little over one time in a second. So this is definitely not right. Mark pedals his bike about once every hour. Well, an hour is 60 minutes. So if he does 70 times in a minute, he's going to do 60 times 70 in an hour. So this is also crazy, that he only does once in an hour. This is unbelievably slow. Mark pedals his bike about once every second. Well, we've already reasoned that there's 60 seconds in a minute, and he does 70 times in a minute. So he does a little over-- if you divide 70 by 60, so you could divide 70 by 60, and you're going to get a number that's a little bit over 1 time per second. So this is right. We're estimating. We don't get exact numbers. It's about once every second. So that seems legitimate. Mark pedals his bike about 4,000 times every hour. So let's think about that. If he pedals 70 times in a minute, and then there are 60 minutes in an hour, this is going to be equal to 70 times 60. Well, we can multiply the 7 times the 6, and we get 7 times 6 is 42. And we have two zeroes here. So we have exactly 4,200 times every hour. So as an estimate, this is pretty close. So I would go with that as well.