SOLUTION: 100 more than greatest 3-digit number is

Let's see now.  The greatest 3-digit number is 999, because if
you add 1 to that you get 1000, which is a 4-digit number. So
999 is the greatest 3-digit number

"100 more than" means to add 100 to it.  So that's just 
what we will do.  We'll add 100 to 999:

        999
       +100
       1099

So the answer is 1099.  It didn't really take a rocket scientist,
now did it?

Edwin