Sit & Solve Cross Sums

I love cross sums, and I love this book. The puzzles are just hard enough to really make you think before you can complete them. Perfect!!

A cross sum puzzle is similar to a crossword puzzle, in that it is formed by putting together interconnected rectangular sections that are across and down. However, for a cross number puzzle, the entries are digits rather than letters. Instead of clues for words, the parameters are the sum of the digits in the rectangular sections. Furthermore, no digit can appear twice in a particular horizontal or vertical section. The approach to solving these puzzles is similar to that used in solving crossword puzzles. First identify a location where there are few options. For example, when there are only two digits and the sum is 15 or greater, then there are at most two options for the pair of numbers. Without such a reduction in the possibilities, there is a combinatorial explosion that can be overwhelming. I found the puzzles in this book to be challenging, but not overwhelmingly so. The restrictions are loose enough to create difficulty and tight enough to make it reasonable. By first scanning the entire puzzle searching for the sweet spot where the restrictions are greatest, I was able to solve most of the puzzles in a short time. In some cases, I found it necessary to resort to algebra in order to represent and understand the options.