This picture is taken from the Problem Pictures resource of photographs with activities for mathematics teaching.
If you purchase this resource the photographs are available in higher resolution and there are additional pages containing hints and solutions.
This plate is designed to help you cut a round cake or pie into equal pieces. Imagine that you place your cake in the middle of the plate. If you wanted nine pieces, you would cut at all the positions marked "9" and also at the position marked "0".
The plate is called a plat diviseur and comes from Brittany in France.
The plate lets you divide into 3, 5, 6, 7, or 9 pieces. Why do you think 2, 4, and 8 have been left out?
Make your own design for a dividing plate.