Answer:
This is because as you add and subtract the numbers, you are really just moving them around nonsensically, to leave you back at the original two numbers. the same thing works with phone numbers. I do this sometimes as a magic trick.
Explanation:
14x2
28+5
33x50
1650+15+365
2030-615
=1415
This works because most of the numbers are already set, so there are only two number that change, the math problems work to ensure that you reach the point where only the two variables are displayed. this also works with phone numbers.