How to Use Factors as an Informal Approach to Multiplication

Hi, I'm Rebecca. And in this film, we're going to look at one approach for multiplying mentally using factors of numbers. And the purpose of using factors is to do what looks like quite a difficult multiplication in a much easier way. So, first we have a calculation here. 16 times 15. And we know that 15 is a number that can be easily doubled or multiplied by two. 16 is an even number, so one of its factors must be 2. And we know that the factor pair that makes 16 is 8 and 2. So 2 times 8. So we've turned our calculation into something quite different. Now we now have 2 times 8 times 15. Now, that's not actually looking any easier at the moment. So you'd like to think that pupils at this stage would understand that the order of the numbers in this multiple multiplication don't matter. They still get the same answer. So what we can do is we can turn the calculation into 15 times 2 times 8. And now it's a calculation that people can solve much easier. 15 times 2 is 30, and 30 times 8, well, we know 3, 8 are 24, 30 is 10 times bigger. So 30 times 8 is 240. We solved that much more easily. Let's have a look at another one. 25 times 16. Well, we know that the factors of 16 are 4 and 4. 25 times 4 is 100 and 100 times 4 is 400. So there's one very easy method for multiplying mentally.