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As a puzzle solver, you can select the numbers from the drop-downs (figure 1) and then press either of the "Solution" buttons to see solution(s) in the area below the calculator (figure 2). There may not be any solutions or there may be many (figure 3).
As a game, you can press either of the "Random" buttons to get a game. These are guaranteed to have at least one solution. When you have a solution you can check it using "Calculator" (figure 4) or the "Solution (all)" button.
As a trainer, use "Calculator" and "Solution" to explore number operations and the various techniques of solving the puzzles.