Lesson video

Hi I'm Mr. Chan and in this lesson we're going to learn about increasing and decreasing by a fraction of an amount.

Here's an example I'm going to increase 20 by 3/5.

So the 20 represents the whole and I need to think about splitting the 20 up into five equal parts to represent fifths so here's my fraction bar model.

Five equal parts, that's the 20 and in terms of trying to increase by 3/5 that means adding those 3/5 onto the 20.

So I've added those fraction parts to the end of the 20 there, now in order to find what 1/5 is I divide 20 by five that equals four that tells me that each part, each fifth is four, so I put that into my fraction model.

And think about what is the answer now, what is the total I can see in my fraction bar model.

Well I can count that there are eight parts, so I would multiply that by eight each part being four, that gives me an answer of 32.

Here's an example where I'm going to decrease 35 by 2/5.

So I've got my fraction bar model there to represent fifths so that's five equal parts when the whole part would represent the 35.

Now what I'm really looking for is that part there because I'm decreasing by 2/5 so the 2/5 something at the end we don't really want.

So in order to find 1/5 I would do 35 divided by five that gives me seven.

So each fifth would be seven.

Now there's several ways we could calculate the final answer but a common method is to think about subtracting the 2/5 at the end there, 2/5 I can see is 14 because that's two lots of seven, so my final answer would be 35 subtract 14 which gives me 21.

So the final answer I'm looking for is 21.

Here's an example for you to try.

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

Here's the answer.

In this question I started off by finding out what 1/5 of 35 was and that would be seven because I'm increasing by 2/5 so I added two parts onto the end of the 35 fraction bar model.

To give a final answer I calculated seven multiplied by seven 49.

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.

If you look at Beth's working out in this question what she's actually done is found 1/11 and then found 2/11 by multiplying that amount by two, so all that's left for Beth to do now is to subtract those 2/11 from the 55 to give her final answer.

So you can see her final calculation is 55 subtract ten which equals 45.

Here are some more questions for you to try, pause the video to complete the task, resume the video once you're finished.

Here's the answers.

Please take care when reading the question whether it's an increase question where you would be adding on the fractional amount or a decrease question where more commonly you would subtract the fractional amount.

And also, don't forget to include units with your answers.

Here's a bit of a problem for you to solve.

Can you find which store is the TV cheapest in? Pause the video to have a go, resume the video once you've finished.

Here's the answer.

So in this question Emily's trying to find the cheapest TV from two different stores.

The normal price is cheaper at TV me TV you, but when you decrease by the fractions given, what you do find is that at TV World, the TV will cost 300 pounds and at TV me TV you it would cost 310 pounds.

So given the fraction decrease it's cheaper at TV World.

That's all for this lesson, thanks for watching.