Take these two digit numbers: $$xa$$ $$yb$$
Assume $$x>y>0$$ and $$b>a>=0$$ and we are to subtract the two numbers. If for example we set, a=3 and b=6, the unit digit of this subtraction will always be 7, regardless of what x and y are. I want to know if there is an identity that can help you identify the unit digit of a subtraction or a shortcut that can help you realize the third number by simply glancing without a thought. I would really appreciate your help.