Lesson video

- Hi, I'm Mr. Chan.

And in this lesson we're going to learn how to find the whole when we're given the fraction of an amount.

So we're gonna begin with two examples.

So these two examples look very similar but they are very different.

So we're gonna begin with what is 3/5 of 75? Well, in terms of fifths I know I need a fraction model with five equal parts.

So to find 3/5, I know the whole is 75, I need to find 3/5.

I'm going to divide 75 by 5 to equal 15.

That tells me what 1/5 is.

So I'm filling in the fraction model with each fifth being 15.

Now, in order to find 3/5 now, I would just multiply 15 by 3 to equal 45.

So the three parts there would equal 45.

So now I know that 3/5 of 75 is 45.

In this next example we're actually we're told that 3/5 of a number is 75.

What is the number? So again, we're dealing with fifths.

So there's my fraction model where I'm told in this case that 3/5 is 75.

So this is how the fraction model differs from the first example.

And I need this time to find what is the number.

So I need to find the whole part.

Again, I'm going to proceed by trying to find what 1/5 is.

So we can see that the 75 represents 3/5.

So I'm gonna divide by three in order to find 1/5.

That's 25.

So now I know that each fifth is 25.

In order to find the whole number, I would multiply the 25 by five to give me 125.

So that would be what the whole represents.

So the actual answer to the question, the number is 125.

Here's another example.

We're told that 2/7 of a number is 18 and we're asked what is the value of the whole number? So when I'm thinking about fractions of sevenths, I'm going to have a fraction model with seven equal parts.

So there are my sevenths.

I'm told that 2/7 is 18, and I need to find the whole number, which is all all of the fraction model.

Now, in order to find 1/7 in this example, I would do 18 divided by two.

That equals nine.

And that tells me what each seventh is.

So each seventh is nine.

Now in order to find the whole, I know that there are seven sevenths in a whole.

So I would do the nine multiplied by seven, which equals 63.

So my final answer is that 2/7 of the number is 18.

So my whole would be 63.

Here's a question for you to try.

Pause the video to complete the task.

Resume the video once you're finished.

Here's the answer to the first question.

In this question we're told that 3/7 of a number is 15.

So I started by finding what 1/7 is by doing 15 divided by three.

That gave me the answer of five.

So in order to find the whole number that gives me five multiplied by seven to get 35.

Here's another question for you to try.

Pause the video to complete the task.

Resume the video once you're finished.

Here's the answer.

Mark's mistake here is dividing by five.

I don't think Mark's read the question properly because he's may be trying to find what 1/5 of 30 is.

But what mark should be doing, because he's been told that 2/5 of the number is 30.

That represents two parts out of the five, Mark should start by dividing the 30 by two.

Here are 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 for question three.

How did you do? If you didn't quite get the all right, it might be worthwhile reviewing some of the examples at the beginning of the lesson.

Well done if you did get the all right.

Here's a problem that you can try and figure out.

Pause the video to have a go.

Resume the video once you're finished.

Here are the answers.

In this question, you're told the special offer of a fraction off the normal price.

So if we look at GetShirt, the special offer is 2/5 off the normal price, which gives you a sale price of £36.

Think about it this way.

The £36 represents 3/5 of the original price.

So if you know what 3/5 of the original price is, you can then work out out what the original price the whole amount was to start with.

That's all for this lesson.

Thanks for watching.