The total ratio for the cups of sugar and water needed to make lemonade is 1+2 = <<1+2=3>>3
In the 3 cups needed to make one liter of lemonade, the fraction representing the cups of sugar needed is 1/3.
If she used 84 cups while making lemonade, she used 1/3*84 = <<84*1/3=28>>28 cups of sugar.
#### 28