Here’s a multiplication investigation that I used with a mixed-age Y5/6 class. Before the task we recapped on the grid method (it will become apparent why the grid method was used). Then, the main task was introduced:
Immediately, the children positioned 8 in one of the tens columns and 0 in one of the ones columns. Some children tried 85 x 40, but then we established that the larger digits (8 and 5) need to be placed in the tens columns and the smaller digits (4 and 0) in the ones columns. But where, then, should the digits 4 and 0 be positioned? Which gives the larger product: 84 x 50 or 80 x 54? Will these two calculations give the same answer? This was explored:
We compared these two calculations and asked ‘What’s the same? What’s different?’ The grid method helps to show that multiplying the tens values gives 4000 in both calculations. However, the TO x O parts of the calculation are different. We saw that 80 x 54 gives the largest product. One child even said ‘The short method for multiplying is quicker, but the grid method is better for showing what’s the same and what’s different.’ I show the children an area model to help them to understand the difference between 84 x 50 and 80 x 54:
In the image above, we see the TO x TO part of the calculation. We ask the children to visualise how each image will be changed when the TO x O part of the calculation is also shown (see below):
Of the two calculations, 80 x 54 gives the larger product because we are multiplying a larger tens value by the 4. I also commented that the pair of numbers that are relatively closer together gave the larger product. Then we look at the examples below, comparing pairs of numbers with the same sum that are multiplied:
We see that the pairs of numbers which are closer together have the greater product.
I have run a very similar investigation using a TOxO calculation. Here’s an opening prompt:
And it led into the I See Problem-Solving investigation below. It’s one of the free sample tasks that can be found here:
If you have a go at these investigations or something similar, I’d love to hear about how you get on!
I See Problem-Solving – UKS2 and I See Problem-Solving – LKS2 give a huge bank of rich tasks. There are Worked Examples and Support, Explain and Extend features that help to deepen children’s understanding.
Information can be found here about INSET/Twilight training on developing reasoning and problem-solving. Bookings are currently being taken from late February onwards. For very cost-effective training, you may consider hosting a training event, or running example lessons in your school.