Question 1077392
I would use the example of speed in flat water and in a current.  There are two variables, and if one is going downstream, the two are added, and if one is going upstream, the current is subtracted.  By having two equations in two unknowns, we are able to determine what each unknown is. 
x+c=8
x-c=2
2x=10
x=5
c=3