There will be an equation, an equality to be specific, in which the user will know 3 of the 4 pieces of information. In real life, it would look like this:
x 10
-- = --
50 100
Since the user is trying to find the missing piece of information, they imput "x".
Now, in the program, they imput the data they know, and put in "x" for the missing piece they don't know.
cout<<"Quadrant I"<<endl;
cin >>a;
cout<<"Quadrant II"<<endl;
cin >>b;
cout<<"Quadrant III"<<endl;
cin >>c;
cout<<"Quadrant IV"<<endl;
cin >>d;
The program then takes the imput variables, makes sure that there is atleast one x and only one x (lines 20 & 24), then depending on if a, b, c, or d is "x", it will preform a mathematical operation on the code.