Instructions for Cross Sum puzzles

Objective

In these puzzles, you are filling the white squares with digits 1 through 9 so that they add up to the sums shown in the dark squares. A dark square will show one or two sums, separated by a diagonal line. A sum that is above the diagonal is the total of the row of white squares to its right. A sum that is below the diagonal is the total of the column of white squares beneath it.

Two important rules:

• There are no zeroes.
• There are no repeats, i.e. a digit can appear only once within a sum.

Entering Digits

Click on a square to highlight it, and enter a digit using the keyboard. You may also use left, right up and down arrows to move about the white squares of the puzzle. Use Check Answer when you are finished. Missing or incorrect squares are highlighted in red.

How to Solve

The "no-repeat" rule usually reduces the number of possibilities to a few you can solve through inspection or trial and error.

• Start at the edges first. Leave big open areas until the end, when you have solved the squares that lead into them.
• Look for the intersection of short rows and short columns. Often, only one digit will work in both.
• Look for "extreme" sums. For example, the only way to make 24 with three digits is some arrangement of 9, 8 and 7. And the only way to make 23 with three digits is some arrangement of 9,8, and 6. At the other extreme, the only way to add up to 6 (or 7) with three digits is some arrangement of 1,2,3 (or 1,2,4).
• Look out for intersections of a "high-extreme" row with a low-extreme column (or vice versa.) For example, if a column of five digits adds up to 16, it must be some arrangement of 1-2-3-4-6. A row of four digits that add up to 30 must be some arrangement of 9-8-7-6. The digit in the intersection of row and column must be a 6, because 6 is the only digit in common.
• Sometimes, you will have to resort to trial and error. Use a pencil, writing small digits in the corners of a cell. Often, you will not get very far before you create a contradiction to eliminate a possibility.

The above rules will get you going for most puzzles. When you are ready to move on to harder puzzles, read up on advanced solving techniques.