This 80/100-21 (2.75/300-21) inch inner tube (innertube) is most commonly (though not exclusively) associated with 150cc, 200cc, 250cc and higher displacement dirt bikes / pit bikes with a 19 inch rear / 21 inch front wheel configuration.
 
Commonly associated denotations:
2.75-21
70/100-21
80/80-21
80/90-21
90/80-21
26x3/300-20