You could easily Test Answers on this question by plugging the answer choices into the given information. When working backwards from the answer, I recommend a) starting with the easiest number, or b) if they all seem easy to work with, start in the middle with answer choice C. That way, you can easily decide whether to go up or down from there.
C) 50
50 cans concentrate means (50)(12) = 600 ounces concentrate
150 cans of water means (150)(12) = 1800 ounces of water
600 + 1800 = 2400 ounces total
We need 200 6-oz servings, in other words 1200 ounces total. 2400 is twice the amount that we need, so we need to go lower. But since it's exactly twice the amount we need, let's try half that amount of concentrate...
A) 25
25 cans concentrate means (25)(12) = 300 ounces concentrate
75 cans of water means (75)(12) = 900 ounces of water
300 + 900 = 1200 ounces total
This is the amount we need, so A is the correct answer.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education