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Unit reasoning | Worked example

Sal Khan works through a question on unit reasoning from the Praxis Core Math test.

Video transcript

- [Instructor] We are asked which of the following is the most reasonable estimate of the mass of a laptop computer? Pause this video, and see if you can answer this. All right, now before we do it together, I'll give a little bit of a disclaimer. They're asking for mass, and in everyday language people often use mass and weight interchangeably. Well, if we're thinking in a physics or an engineering context, we have to be much more clear about whether we're talking about mass or weight. But in this context, let's just assume that we're using the more everyday language version of mass. And so even though a physicist or an engineer in a lot of context might consider ounces or pounds weight, not mass, we'll just think about it in terms of mass for the sake of this question. So you could try to visualize a laptop. Think about how it feels on your lap when it's sitting there. If you're used to thinking in pounds, you might say, well, yeah, it'd be several pounds. Even a light laptop is going to be one or two or three pounds. A heavy laptop might be 10 or more pounds. So that gives you a sense of things, but now let's just look at these different choices and try to think about it in the world that we live in. So this first one is 5/100 of a metric ton. So just as a review, a metric ton is 1,000 kilograms, so we could rewrite this as 5/100 times 1,000 kilograms. And so what would this be? If you divide both 1,000 and 100 by 100, you get five times 10. This is equal to 50 kilograms. Now if you don't have an intuition of what 50 kilograms is, many in the U.S. would not, a kilogram, one kilogram is approximately equal to 2.2 pounds. So this over here would be over 100 pounds, which would be ridiculous for a laptop. That would make your thighs hurt. So you'd rule that out. Here we have half an ounce. Just as a reminder, one pound, one pound is equal to 16, 16 ounces. So this is half of a 1/16 of a pound. Laptops are getting lighter every day, but they aren't this light. This next choice is five pounds. Well, this seems to match our intuition that a laptop would be several pounds, so this one is looking pretty good. I like that one. But before I circle it in, let me look at the other choices. 50 kilograms, well, that was the same as choice A, just written it as a different way. That's over 100 pounds, so I would rule that out. 500 grams, well, this is the same thing as 0.5 kilograms, which would be approximately, if you wanted to think about it in terms of pounds, it'd be approximately 1.1 pounds, if you just take the half of both sides of this. And although there are laptops that have already gotten below two pounds, this would be an unusually light laptop, but I am not even aware of any laptops that are this light just yet. So I would rule this one out, although this one would be a close second. Maybe if I did this video in a few years, F might be a better choice than C. But in the world we live in today, C is the best of these choices.