A short instructional transcript in which Rebecca explains what a multiple is and how to find the lowest common multiple (LCM). She demonstrates with three worked examples: LCM(5,40)=20; finding LCM(8,7)=56 to compare 6/8 and 5/7 (showing 6/8 is larger); and a packing word problem with packs of 4 and 6 people (LCM=12, so buy three packs).
Chapters
Hi, I'm Rebecca. And in this film we're going to look at how to find the lowest common multiple. So let's start by defining what we mean by multiple. A multiple of a number is where you multiply two numbers together and get the answer. Basically, it's the numbers in a times table. So let's have a look at how this works. Here we want to find the lowest common multiple of 5 and 40. So I've listed the multiples of 5, the multiples of 4, and you can see that there are two in common. The multiples of 5, we have 5, 10, 15, 20. And then with the multiples of 4, we have 4, 8, 12, 16 and 20. They also have 40 as a common multiple, but we're looking for the lowest common multiple. So they both have 20 in common and that is their lowest common multiple. We can also use this strategy to compare fractions. Here we're asked what is larger, 6, 8 or 5/7. But we need to have a common denominator to compare them. So we can find the lowest common multiple of eight and seven. So here I have the multiples of eight written out and the multiples of seven. And you can see that between them they have a common multiple of 56. So I'm going to convert both fractions into 56. So we have 6, 8. I've multiplied the denominator by 7, so I need to do 6 times 7, which would be 42, 56, and then the same with the 5, 7. This time I need to multiply the numerator by 8. 5 times 8 is 40, so I have 40, 56. So we can see that 6, 8 is actually the larger fraction. We can also use the lowest common multiple to solve word problems. Here, cans of lemonade are sold in packs of four, and I want to buy enough to share them equally between six people so there are no remainders. What is the smallest number of cans I can buy? So here my key numbers are 4 and 6. So I need to know what the multiples of 4 and 6 are to be able to answer the question. So here multiples of four are written out and so are six. And we can see that our common Multiple here is 12. If the cans come in packs of four, then we need to buy three packs. So there you go. That's finding the lowest common multiple.
