Lesson video

Hi, I'm Mr Chan.

And in this lesson, we're going to learn about finding the fraction of an amount.

Here's an example.

I want to find one fifth of 20.

Finding one fifth means I need a fraction model that has five pieces, each of equal size.

So I'm interested in one fifth.

So that's the part I'm looking for.

The whole is represented by the 20.

And I need to find out what that one fifth part is.

So to do that, I would divide 20 by five, that equals four.

So each part in that fraction model is four.

So the answer is four.

I only wanted the one part.

So one fifth of 20 is four.

Here's another example.

I need to find three sevenths of 42.

So I'm going to start off with my fraction bar model with seven pieces, and I need to find three parts of that.

So I'm trying to find three sevenths.

The whole part is represented by the 42.

So I need to figure out what those three parts are.

So I start off by finding what one seventh is by doing 42 divided by seven, that would equal six.

So I'll fill in my fraction bar model, each part, each seventh is six.

Now in order to find what three sevenths is, I just do three times six, that gives me 18.

So the final answer three sevenths of 42 equals 18.

Here are two questions for you to try.

Pause the video to complete the task, resume the video once you finished.

Here are the answers.

So in question one, you're finding three fifths of 15, and there's a fraction bar model drawn for you that have five equal parts.

In question two, there hasn't been a fraction bar model drawn for you.

Can you think in terms of working out three fifths and then working out two sevenths? Could you answer the question without a fraction bar model? Here's some more questions for you to try.

Pause the video to complete the task, resume the video once you're finished.

Here are the answers.

These questions are asking you to find the fraction of an amount where the amount is actually representing a quantity of something.

So if we look at part a, for example, you're asked to find two thirds of 600 kilogrammes.

Now your answer should also be in kilogrammes.

So make sure that you'll include units in your answers for this question.

Here's an example where we have to compare two fractions of an amount.

So I'm comparing five sixths of 48 kilometres with six tenths of 80 kilometres.

And I need to put a symbol inside the box, either a less than symbol, a greater than symbol, or maybe an equal symbol, if they will equal to each other.

So, in order to find five sixths of 48 kilometres.

First, I imagine a fraction bar model with six equal pieces.

So that would mean that I have to divide 48 by six to get eight, and I want five sixths, so I would multiply that amount by five, so that would be five times eight, that amount is 40.

So five sixths of 48 kilometres is 40 kilometres.

I would do the similar method with six tenths of 80 kilometres.

I would imagine a fraction bar model with 10 parts.

So I would divide 80 by 10, that gives me eight.

I only want six parts of that fraction bar model.

So I would multiply that by six.

So six multiply by eight is 48.

And with these two fractions of an amount, I can see that five sixths of 48 kilometres is less than six tenths of 80 kilometres.

So I would put the less than symbol there.

Here's another example.

I've got to compare two thirds of 33 litres with two ninths of 81 litres.

So let's figure out what two thirds of 33 litres is first.

I need to think about splitting 33 litres into thirds.

So I'm thinking about a fraction bar model with three parts.

So I would divide by three to find one third first, that would equal 11.

I only want two thirds though, So I would multiply by two, two multiplied by 11, gives me 22.

So let's find two ninths of 81 litres now.

So I would imagine splitting 81 up into ninths.

So I would divide by nine to find what one ninth is that would give me nine, 81 divided by nine is nine.

And I only want two ninths, so I would multiply that by two, two multiplied by nine is 18.

So I've got the two amounts to compare now.

I can see that 22 litres is actually greater than 18 litres.

So the answer I would put the greater than symbol into the box.

Here are some fraction of an amount comparison questions for you to try.

Pause the video to complete the task, resume the video once you're finished.

Here are the answers.

In this question we're comparing two fractions of an amount with each other.

So in part a I found three fifths of 85 kilometres, I worked out to be 51 kilometres, compared that with two thirds of 120 kilometres, that was 80 kilometres.

So I know a 51 is less than 80.

So I wrote the less than symbol for part a.

Hopefully you got the others correct as well.

Here's a question for you to try.

Pause the video to complete the task, resume the video once you're finished.

Here are the answers.

In this question Will is working out five sixths of 30.

So that involves splitting 30 up into six equal parts, and dividing by six to find what one part is.

And that's where Will has made the mistake.

Will has divided by five and he should have divided by six.

That's all for this lesson.

Thanks for watching.