Bob uses 2/5 of a cup of vinegar in his salad dressing recipe. how much vinegar would bob use to make 2 3/4 recipes?

Alls you would have to do in this real world problem is take the amount of vinegar it took to make 1 recipe, and multiply it by that of the new amount of recipes Bob wants to make. 
2/5 * 2 3/4 = 2/5 * 11/4 = 1.1 Cups of vinegar

Ithink if ur looking for a ratio it would be 12: 3