In the first Mishna on Mesechta Bava Kama Daf סא עמוד ב (https://www.sefaria.org/Bava_Kamma.61b?lang=he), there is a מחלוקת about how far does a fire have to pass in order to be פטור.
When רבי עקיבא states his opinion he says "50 אמה", not "50 אמות? Why does he not write "אמה" in plural form?