Video: GCSE Mathematics Foundation Tier Pack 4 • Paper 2 • Question 6

GCSE Mathematics Foundation Tier Pack 4 • Paper 2 • Question 6

03:22

Video Transcript

One stone is 14 pounds. Three kilograms is approximately seven pounds. Work out an approximation for the number of kilograms in five stone.

We’re told in the question that one stone is equal to 14 pounds, but how are we gonna use that? What’s our next step? Well, what we want to do is actually work out an approximation for the number of kilograms in five stone. So the way we’re gonna deal with this is we’re gonna change five stone to pounds, and then we’re gonna go from pounds to kilograms to give us the number of kilograms in five stone.

Well, the first step to actually convert our five stone into pounds is to actually find out how many pounds are in five stone. So let’s think about one stone equaling 14 pounds. Well, to get from one stone to five stone, we have to multiply by five. So therefore, we have to do the same to our 14 pounds. And when we do that, we’ll get the result 70 pounds.

Okay, so we can do that on the calculator, but how do we do it using written method? Well, we could use formal column multiplication, like this. We got 14 multiplied by five. So first, well, we do five multiplied by four, which is 20. So we put zero in the units column and carry a two into the tens column. And then we do five multiplied by one. Add the two that we carried gives a seven, which will give us 70. Okay, great, so that’s the written method.

Actually, there’s a very easy mental method we could’ve used to work this out, because you could do five times 10 in your head, which is 50, then five times four, which is 20, then add them together: 50 plus 20 gives us 70. That’s cause five plus two is seven, add a zero 70.

Brilliant! Okay, so that’s just a couple of methods we could wor- use to work that out. So if we check our method, well, this is the first stage complete, because we’ve actually changed five stone to 70 pounds. So now the next thing we need to do is actually go from pounds to kilograms.

Well, to enable us to do that, we’re gonna look at this bit of information here. We’re told that three kilograms is approximately seven pounds. So therefore, what we want to work out is how many lots of our three kilograms are gonna go into our 70 pounds or our five stone.

So to work that out, we’re gonna divide 70, so 70 pounds, by seven, because we know there are seven pounds in three kilograms. And when we do that, we get 10 as our result. So we know that there are 10 lots of our three kilograms in five stone or 70 pounds. So therefore, to work out the last stage, which is to convert from pounds to kilograms, we need to multiply our 10, because we said there are 10 lots of three kilograms in 70 pounds, by three kilograms. And when we do, we get 30 kilograms.

So therefore, we can say that we’ve actually completed the final stage, which is converting from pounds to kilograms. So we can say that if one stone is 14 pounds and three kilograms is approximately seven pounds, then an approximation for the number of kilograms in five stone is approximately 30 kilograms.

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