First time caller

[Guest]

Out of my depth but vaguely interested. Maths teacher told me that he could prove that it's impossible to finish a 100m race due to infinity! His point was that at 99m (or any point before or after) the remaining distance could be extrapolated to infinity using tenths/hundredths/thousandths etc of a metre thus making it impossible to actually cross the finishing line! I KNOW they do actually cross the line but I can also see his point. Please explain without hurting my brain, or ridicule?
Regards thick plumbing and heating engineer