Two whole numbers differ by $$1$$. If one number has $$3$$ digits and the other has $$4$$ digits, what is their sum? 
Overseas Competition->Knowledge Point->Number Theory Modules->Place Value and Number Bases->Numbers
If $$1$$ more than a $$3$$-digit number is a $$4$$-digit number, then the numbers are $$999$$ and $$1000$$ and their sum is $$1999$$. 