I can follow the algorithm set that’s been proposed for solving “must” problems. However, I still can’t wrap my head around why this algorithm works. In the attached screen shot, it says that option B is correct, (i.e., the price MUST be less than $13). However, I don’t get how this is possible, as there are counterexamples that show it doesn’t need to be less than $13.

Hi @blam3, it’s Orion, the author of Achievable’s GRE content, here to answer your question. It’s a good one!

The strategy I teach for Must problems (the “algorithm”) isn’t technically true (i.e., it won’t always lead to the correct selection in all cases), but it’s practically true (i.e., it will always lead to the correct selection in all cases on the GRE ). The reason why this strategy will always work on the GRE is because the values in the answer choices should always be keyed in the “non-obvious” direction. This is because if the answer choices were keyed in the “obvious” direction, it would be very easy to identify counterfactuals and the difficulty of the problem would plummet.

The issue with the particular problem you identified is that this doesn’t happen (i.e., the answer choices A and B are keyed in the “wrong” direction). As a result, we’ve flagged this template for review and are addressing the issue. So it looks like you found a typo. Nice work!

Hopefully this explanation makes sense. Let me know if you have any other questions, and thanks for using Achievable!