Automated subtraction

[Ummoo]

Hi, 

I want to apply a condition / or any solution without using a third cell to automate and reduce (Subtract) the amount (number) from B1 by the value of A1. 

There is value in B1, I want to put a Value in A1 and want it to be reduced in B1, BUT I don't want to use a third cell for it or Tables. anyone to help? 

[#2]

This makes no sense. Where does the value in B1 come from? Someone must have entered it. But if your definition for B1=x-A1, when someone enters "10<ENTER>" in B1, he would see 10-A1 pop up. That would throw any user. Unless B1 is a hard-wired constant, in which case your formula in B1 would simply say =10-A1.