Genaille–Lucas rulers

Genaille–Lucas rulers are an arithmetic tool used to multiply any number by a single digit number. Align the rulers that make up the first number to the right of the index ruler, then follow the series of triangles starting from the line corresponding to the second one-digit number and read off the product obtained. If you want to multiply a number containing the same digit multiple times, you have to print the same ruler multiple times.
For more information, here's a wikipedia page: https://en.wikipedia.org/wiki/Genaille%E2%80%93Lucas_rulers and here's a video that shows how to use them: 

I would suggest to lower the extrusion width of the perimeters and the solid infill to at least 0.4mm. Printing the last layers with a different color enhances contrast.