If we divide the numerator and denominator of a fraction by the same number we get the same number just in a different form.
If we divide the denominator and numerator of 80/100 by 10 then we get 8/10 which is the same as 8/10.
Therefore someone who ran 8/10ths of a mile ran the same as someone who ran 80/100ths of a mile since they are the same.