Understanding the 45 Rule
Killer Sudoku combines logic with arithmetic. The most powerful technique is the 45 rule.
What is the 45 Rule?
Every row, column, and 3x3 box contains numbers 1-9, which sum to 45.
How to Apply
1. Find cages that almost complete a unit
2. Calculate the difference from 45
3. This tells you what remains
Example
If a row has cages summing to 42, the remaining cells must sum to 3.