My solution for this: http://codereview.stackexchange.com/questions/126071/solve-6-simultaneous-equations-for-8-variables-puzzle-with-java

Puzzle.java

import java.util.Arrays;
import java.util.stream.IntStream;

public class Puzzle {
    public static int[] unpack(int x){
        return IntStream.iterate(x, i -> i/10).limit(8).map(i->i%10).toArray();
    }
    private static boolean isSolution(int[] variables) {
        return (variables[5]!=0) &&
            (variables[0] + variables[1] - 9 == 4)
            && ((variables[2] - variables[3]) * variables[4] == 4)
            && ( variables[5] + variables[6] - variables[7] == 4)
            && ((variables[0] + variables[2]) / variables[5] == 4)
            && ((variables[1] - variables[3]) * variables[6] == 4)
            && (9 - variables[4] - variables[7] == 4);
    }
    public static void main(String[] args) {
        IntStream.range(1,100000000)
        .parallel()
        .filter(x->isSolution(unpack(x)))
        .forEach(x -> System.out.println(Arrays.toString(unpack(x))));
    }
}