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Read the Sudoku help page for the rules, and the techniques potentially required to solve BrainBashers Sudoku.
Forced Moves is one of the basic techniques people use when solving Sudoku.
When there is no choice (because the other digits already appear in the row, column and block), a square is forced to be a particular digit.
R5C2 can only be a <2>, as no other possibilities are allowed.
R1C4 can only be a <7>.