A recurring theme in math is that you may not need to do what it looks like you need to do. There may be a shortcut to where you want to go. A special case of this is that you may not need all the information that you think you need.
For example, if you need to know the last digit of a × b, it might seem like you need to know a and b so you can multiply them together. But you only have to know the last digits of a and b. In fact, if one of the last digits is 0, that's all you need to know.