A3- pack of boxes of juice costs $1.12. a 12-pack of boxes costs $4.38. a case of 24 boxes costs$8.68.which is the best buy?

First, you divide the price by the amount of units in the pack. This is how you find how much one unit is worth in that pack. So for the first pack you divide 1.12 by three, and then you have .37333 repeating, which is about 38 cents. Next is the second pack, where we divide 4.38 by 12 which is .365, which is about 37 cents. The last pack is 8.68 divided by  24 which is .361, or about 36 cents. This means that the 24 pack is the best buy.
In real life you should also consider which would be too much for you or not enough. For example,  you might not even need 24 units of juice. You might only need 12 so you don't have too much extra. But hey, this is just a math problem ;)