This applet provides an opportunity to pose questions that have more than one correct answer.
If one of the numbers - top, bottom or sum is hidden, then clearly there is one correct solution to the problem.
If two of these numbers are hidden - there are many solutions to the problem. How many?
If selected digits are masked then each problem has a finite number of correct solutions.
Can you find a solution?
How many solutions can you find?
How do you know you found them all?
Best viewed as a Java applet