Greatest is <30000.5 but >30000, and least is 29999.5. The greatest can't be identified exactly. 30000.4995 rounded to the nearest whole number is 30000, but so is 30000.4987654321 and many others. This is the rule of rounding. The best way to express it is ~<30000.5 or, in words, just less then 30000.5, as close as possible to 30000.5, because by the rules 30000.5 itself rounds to 30001.
You can also round numbers to the nearest ten, hundred, thousand, etc., but the question doesn't specify. For example, 30499 rounds to 30000 and so does 29500 if rounding to the nearest 1000 is required.