Video: Choosing the Suitable Unit to Weigh an Item

Which of the following is closest to the weight of a tennis ball? [A] 20 kg [B] 25 kg [C] 1/4 kg [D] 50 kg [E] 30 kg

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Video Transcript

Which of the following is closest to the weight of a tennis ball? 20 kilograms, 25 kilograms, one-quarter kilogram, 50 kilograms, or 30 kilograms.

In this question, we need to estimate the weight or the mass of a tennis ball. We know we need to estimate to find the answer because the question asks us to find the measurement that is closest to the weight of a tennis ball. It doesn’t ask us the actual amount, but the amount that is nearest. So we need to use our knowledge of measurements in real life to estimate the amount.

The first thing we can say about our five possible answers is that they’re all written in the same unit of measurement. They’re all a number of kilograms. This makes it easier to compare them if we need to. The second thing that we can notice about our possible answers is that four of them are above one whole kilogram. In fact, there are a lot more than one whole kilogram. They’re all 20 kilograms or more. There’s only one measurement that is less than a kilogram, and that’s one-quarter kilogram.

Now, to estimate the answer, we need to use our knowledge of the real world. First of all, what do we know about tennis balls? Well, amongst the other things, they’re made of rubber, which makes them easy to bounce and to hit. They’re filled with air, which, of course, makes them light enough to throw in the air and to hit and so on. We know that the whole aim of the game involves hitting the ball over the net and pass to your opponent. So the ball has to be fairly light for us to be able to do that.

The other real-world fact that we can think about to help us estimate the answer is to understand what one kilogram is worth. Sugar is often sold in bags of one kilogram. So it’s useful to visualize a bag of sugar when we’re thinking about how heavier kilogram is. We can see now how a lot of our answers are really silly. 20 kilograms is the same as 20 bags of sugar. Imagine hitting a tennis ball that weigh the same as 20 bags of sugar. In the same way, 25 kilograms, 30 kilograms, and even 50 kilograms are far too heavy for a tennis ball.

Using everything we know about tennis balls and kilograms, the best estimate is the one that is less than a whole kilogram, one-quarter of a kilogram. Out of our five possible answers, the measurement that is closest to the weight of the mass of a tennis ball is one-quarter of a kilogram.

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