Find the whole when given a fraction of an amount

﻿Hi, I'm Mr. Chan. And in this lesson, we're going to learn about application of fraction of an amount skills. Let's begin with this example. We're told that Petra has eaten four-fifths of her 120 grammes bar of chocolate. How much chocolate has she eaten? So if she's eaten four-fifths, well let's think about our fraction model being fifths, so that's five equal parts, that represents her chocolate bar. So she's eaten four fifths of that so we can shade 4%, and that's how much she's eaten. So let's figure out what one-fifth is by dividing by five, so each fifth is 24 grammes. So in terms of figuring out how much she's eaten, I could figure out 24 multiplied by four which equals 96. So the final answer, how much chocolate has she eaten, Petra has eaten 96 grammes of chocolate. Here's another example. We're told that 85 litres of water fills a bath to five-eighths of its capacity. And we are asked how much water does the bath hold when completely full? So, thinking about the question that we've been given, or the information we've been given, five-eighths fills a bath, so let's think about a fraction model with eights. Five-eighths is 85 litres so let's show that, so five baths is 85, and we're asked how much is the bath when it's completely full? How much water does it hold when it's completely full? So, if we know that five-eighths is 85 litres, we can figure out what one-eight is by dividing 85 by five, so let's do that. 85 divided by five equals 17, so one fifth is 17 litres. In order to think about and figure out how much water does the bath hold when completely full, we can multiply that by eight to give 136. So that tells me that when completely full, the bath holds 136 litres. In this example, we're told that Jilly wants to cut off two-sevenths of her 84 cm ribbon. And we're asked how long will the two pieces of ribbon be once she has cut it? So, let's represent the ribbon that Jilly's got with a fraction model, it's in seven equal pieces because she's going to cut off two-sevenths, so let's show that on the fraction model. She's going to cut off two-sevenths of the end there and what she has in terms of the length of the whole ribbon is 84 cm and she's got some remaining ribbon once she's cut off. So she's going to have two pieces of ribbon once she's cut it. So let's figure out what those two lengths of ribbon will be. So, in terms of figuring out what one-seventh would be, let's divide the 84 by seven, that gives me 12. So each one-seventh of the ribbon would be 12 cm. So now I can figure out how much she's cut off, that would two-twelfths, so 12 multiply by two gives me 24, that tells me that she's cut off 24 cm. And in terms of the remaining ribbon, we could multiply but in this case I'm going to simply subtract the 24 from the total length that she started with, 84 subtract 24 that gives me 60 cm. So she's got 60 cm and 24 cm, those are the two pieces of ribbon that she's got in terms of how long they will be. Here are some questions fit trying. Make sure you read each question very carefully, pause the video to have a go, resume the video once you're finished. Here are the answers. With these questions, take care to read the question fully. So, for example in question one, you're simplifying the fraction of an amount. In question two, you're told what the fraction is and you're trying to find the whole. And in question three, that question is actually trying to find a fraction and then increasing the amount by that fraction. Here's some more questions for you to try. Pause the video to have a go, resume the video once you're finished. Here are the answers. In question four, you're told what five-eights is so you've got to figure out the whole in order to answer the question, and in question five, if the council tax bills are being reduced by or decreased by two-fifths, that means you're finding what two-fifths is and then decreasing by the amount. That's all for this lesson, thanks for watching.

Application of fraction of an amount skills

In this lesson, we will investigate a mixture of problems involving finding the fraction of an amount.

Lesson overview:Application of fraction of an amount skills

Core Content

Fraction Change: