Check below, please
1) Let's find out the sum of 503 + 10 mentally through this modeling:
500 + 10 +3 = 513
That 5 hundred will remain
The Tens place won't change 1
The unit's place won't change.
2) Another model to find out 503 +100 is
500 + 100 + 3 = 603
The hundreds place will add up to 600
The tens place won't change
The units place will change