A simple problem! To find what two numbers create a given number, subtract a number (or add a number). The number you get minus the number you added/subtracted will be equal to the target number. For example:
42000-1=41999
So that would mean 41999-(-1)=42000
You could also do:
42000+1=42001
Then that would mean 42001-1=42000. Hope this helps!
:)