Brandon makes lemon water by adding of a cup of sliced lemon to 2 cups of water. At this rate, how many cups of sliced lemon would he need to add to 12 cups of water?

1 lemon to 2 waters will be proportional to what he Brandon needs so:

1 lemon / 2 water = x lemon / 12 water

Multiply by 12

And you get (1/2) * 12 which is 6=x lemons

He needs 6 cups of lemons