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First of all, I used the clues of the product numbers to work out what factors lower than 9 could fit into the boxes. Once a few of the numbers were filled in, I used these to cancel the chance of those same numbers being placed in another box. After many of the boxes were full by doing so, I figured out which numbers were missing in the boxes of 9 cells, the horizontal rows, and the vertical columns to complete them. After doing so, there were few numbers left to be filled in, so I worked my way down the Sudoku to see where the numbers could and couldn't go.