600
700
600
It is one thousand
It depends on what you are rounding to. 621 to the nearest 10 would be 620 while 621 to the nearest 100 would be 600.
620
621.0
853 rounded to the nearest 100: 900. 10853 rounded to the nearest 100: 10900.
100
600
100
100
0
555 rounded to the nearest 100 is 600.