How to Solve a 5x5 Calcudoku
List the cages with a single answer first: 3+ is 1 and 2, 9+ in two cells is 4 and 5, 4− is 1 and 5, 5÷ is 1 and 5, 20× is 4 and 5, and a three-cell 6+ in a line is 1, 2 and 3. Then use the rule of 15: every row and column adds up to 15, so the cells of a row left outside its cages add up to 15 minus the cage sums, a trick that often fixes a lonely cell. Write candidates small, cross out what each placed digit forbids in its row, column and cage, and remember that a bent cage may repeat a digit while a straight one never does.