Sudoku
Fill every row, column, and 3×3 box with 1–9.
How to play: Fill in the blank squares so every row, every column, and every 3×3 box contains the digits 1 through 9 exactly once. Given numbers are locked. Tap Check any time to see if you've got it right.
Sudoku became a worldwide puzzle phenomenon in the mid-2000s, and it's stayed popular because the rules never change even though every puzzle does. There's no arithmetic involved — just logic and process of elimination. The easy puzzle here is a good warm-up; the hard one holds fewer starting numbers, so you'll lean more on deduction than on the obvious first moves.