In order to solve vertical subtraction, we follow these rules:
First rule - write the problem in the correct order!
Ones digits under ones digits, tens digits under tens digits, and so on.
Second rule - when the upper digit is smaller than the lower digit - we borrow from the next digit.
Third rule - a that cannot be borrowed from becomes until we reach a digit that is not from which we can borrow .
Note! If there is a third right after, it will become , if there is a fourth right after, it will become , and so on.