1 can of concentrate = 12 ounces
3 cans of water = 36 ounces

x = number of cans of concentrate

12x + 3x(12) = 200 * 6
12x + 36x = 1200
48x = 1200
x = 25

You can look at this problem in proportions:

One 12 ounce can yields 48 ounces of juice - 1 can of juice plus 3 cans of water.

To make this problem even simpler, you can notice that the question asks for how many 12 ounces cans of juice are needed for 200
6 ounce servings which is the equivalent of 1200 ounces.

You can now set up the equation to be:

1/48 = x/1200

Cross multiplying you get

1200 = 48x or x = 25