Take the following Killer Sudoku (from the app "Killer Sudoku" by Studio Goya LLC) I have been struggling with for the last few hours. "Penciled" numbers are my notes about which numbers are possible in the cells.
I cannot seem to find the next step. The app I am using says that R1c3 can only be a 5. However, I fail to see the reason. Anything I have tried still allows a 4 to be placed there. What is the (probably very obvious) thing that I am missing here?
For those not familiar with Killer Sudoku rules: Normal Sudoku rules apply. In addition, numbers in cages must sum to the small number in the top left corner of the cages. Numbers cannot be repeated in cages.