Medium Sudoku for Logic Practice, 45 Empty Cells

On the easy level, almost every cell can be worked out directly. On the medium level, with 45 empty cells, that alone falls short roughly a third of the time, and that's where the "hidden pair" technique comes in: if only two cells in a row can hold the digit 4 or 7, you can lock those two cells to that pair, and immediately rule out 4 and 7 from every other cell in that same row, even if those cells technically had three or four other candidates. It's not a guess, it's a hard conclusion: since 4 and 7 must occupy exactly those two cells, they can't appear anywhere else in the row. That's why the medium level is the most popular on the site, the rules alone aren't enough anymore, but it's not exhausting trial and error either, a grid takes 15-20 minutes. Solving quickly and confidently? Move up to hard sudoku. Want more practice with naked singles first? Go back to easy sudoku.