This Cross-Number puzzle involves the use of a novel 'reverse notation' or 'looking glass arithmetic'. Students should be encouraged to investigate and discover rules for adding and subtracting digimirror numbers.A detailed teacher's guide is included which suggests possible classroom approaches. Answers are provided.My resource 'Digimirror Maths' would be a natural introduction to this activity.[Number bases, decimal system, place value, number puzzle, reflection, addition & subtraction of