This would depend on how much you are rounding to. If you are rounding to the nearest hundred then the answer could be a number like 66048. However, if you are rounding to the nearest 1 then the answer could be a number like 66000.4.
There are very many possible answers; some of them depend on the degree of rounding. If rounding is to the nearest thousand, you could have 6581.23047
If a number rounds to 600,000 when rounded to the nearest hundred thousand, it could be any number between 550,000 and 649,999. This is because when rounding to the nearest hundred thousand, any number from 550,000 to 649,999 would round up to 600,000.
800 - 300 = 500
-- If you're rounding to the nearest 100, then it's 750 . -- If you're rounding to the nearest 50, then it's 775 . -- If you're rounding to the nearest 20, then it's 790 . -- If you're rounding to the nearest 10, then it's 795 . -- If you're rounding to the nearest 5, then it's 798 .
1000
It could be rounding to the nearest 110000 (or nearest 770000). Otherwise it is an error.
10000
The answer depends on the extent of rounding. It could be 8995 to the nearest 10 of 9449 to the nearest 1000.
16.13
600
The answer is quite simple. Because 250 is in the middle of 200-300 you can not decide wether to go up or down with the rounding. The answer could be 300 if there need be an answer different from the original number given