This 80/100-12 (3.00x12) inner tube (innertube) is commonly (though not exclusively) associated with 50cc, 70cc, 90cc, 110cc, 125cc, 140cc, 150cc , 155cc and 160cc dirt bikes / pit bikes. Most commonly used as the rear wheel tube in 12 inch rear / 14 inch front configurations.
 
Commonly associated denotations:
80/100-12
2.75-12
2.75/3.00-12
3.00-12