[wutbrodo]

Each pile has three solutions in it, and you're asking how we know that all three solutions align to the same number, as required by Sudoku.

> Starting with each row, Bob randomly chooses one card in each cell, from the top, the middle, or the bottom

The random assignment of solutions from each pile to each of the three problems means that the only way to consistently pass the test is to have the three solutions in each pile be identical.

[kemyke]

Oh, I missed that part, now its clear.