A short YouTube lesson (presented by Rebecca) that teaches pupils how to use compensation to multiply decimals. The video advises first estimating the result (e.g., 15×0.9 ≈ 15), then scaling the decimal by a power of 10 (0.9→9) to perform whole-number multiplication using partitioning (15×9=135), reversing the scale (divide by 10 → 13.5), and finally checking the answer against the initial estimate.
Chapters
Hi, I'm Rebecca, and in this film we're going to look at how we can use compensation to multiply decimals. Now, when pupils see calculations like these, they can tend to panic and we can use compensation to make it a bit more manageable. Changing one of the numbers to something that's easier to work with and then adjusting our answer. In light of the changes that we've made. Before starting, it's really important that you get your pupils to look at the calculation and make an estimation of what the answer is likely to be. Here we can see that 0.9 is very close to 1, and we can round that up to 1 to multiply it to see what our answer is likely to be. Here, if we did 15 times 1, the answer would be 15. So our answer is going to be somewhere around 15. So let's see how we can use compensation to solve this calculation. When working with decimals, it's a really good idea to change them by multiplying them by 10 or 100 so that those numbers are much easier to work with. So here I can multiply my nine by ten, which would give me nine whole ones to work with. Now I'm just multiplying whole numbers. So here I've times it by 10. Now I'm multiplying 15 times 10. You can use partitioning to solve this. So you could do 10 times 9 is 90 and 5 times 9 is 45. Adding them together will give you an answer of 135. Now, with all compensation, you must remember to go back and adjust your answer in light of the changes you've made previously. We multiplied our 0.9 by 10. So now we must make sure that we go back and do the opposite and divide the answer by 10. If you go back, you could say to your pupils, have we got the right answer? Remember, we estimated before that we thought our answer was going to be around about 15. By looking at this, they'll see that we're not there yet. That's not similar to our estimated answer. So here, if we divide our answer by 10, we end up with 13.5. Return to the estimation and you can see that that's a similar answer around about 15. So now pupils know that they've got the right answer. So there you go. That's how to use compensation to multiply decimals.
