Cross-figure

(It is an implicit rule of cross-figures that numbers cannot start with 0; however, some puzzles explicitly allow this) By continuing to apply this sort of argument, a solution can eventually be found.

A curious feature of cross-figures is that it makes perfect sense for the setter of a puzzle to try to solve it themself.

Indeed, the setter should ideally do this (without direct reference to the answer) as it is essentially the only way to find out if the puzzle has a single unique solution.

Dell Magazines publishes a magazine called Math & Logic Problems four times a year that includes these puzzles, which they name "Figure Logics"; the eighteen puzzles contained within each issue generally increase in difficulty, from easy to "challenger".

Some cross-figures voluntarily ignore this option and other "non-mathematical" approaches (e.g. palindromic numbers and repunits) where same result can be achieved through algebraic means.