The greatest whole number to round to 300 would be 449 rounded to the nearest 300
The greatest whole number is 277300.
The answer depends on the extend of rounding. To take an unlikely example, it could be 35549, rounded to the nearest 23700. A more likely answer is 23849 rounded to the nearest 300.
Assuming that you want an integer (a whole number) less than 300, then the number you'll be looking to reach is 299. To get to 299 from 176 you need to add 123.
349
The greatest number is 123 because 300-176 is 124 but since it says less than, it is going to be 123.