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One way to start would be to print off the grid and use tracing paper to make another version of it. Have a think about the different ways that you could place it on top - how many ways of doing this are there? Choose one way and then (without doing all of the calculations) consider which products are going to be the largest. Is there a different way you could turn it to make more or fewer large products?

If you are stuck, you can start with a small grid (perhaps two by two or three by three). Choose a way of putting the second grid on top, and then calculate and add together all of the multiplications. What do you notice?