I am trying to make a rounding function

[gcjr]

what about

#include <stdio.h>

int
main ()
{
    for (int i = 210; i < 300; i += 10)  {
        printf (" %6d %6d\n", i, 100 * ((i + 50) / 100) );
    }

    return 0;
}

which produces

    210    200
    220    200
    230    200
    240    200
    250    300
    260    300
    270    300
    280    300
    290    300