- Fri Dec 18, 2015 7:10 pm
#21347
Hi Kim,
You are absolutely right that (C) would not require a violation of the third rule that Casey cannot buy both a small and a large shirt. So if this were a could be true question with the same local conditions, (C) would be a correct answer choice because it is possible under these conditions that Casey could buy either a small yellow shirt or a large yellow shirt.
But since this is a must be true question, you need to find an answer choice that has to happen. Even with the two local restrictions that Casey cannot buy a large red or a small blue shirt, there are still no restrictions on medium shirts in any color, so it would still be possible to buy three medium shirts (in red, blue, and yellow) and not violate any of the rules. Thus, (C) doesn’t have to happen and cannot be the right answer choice here.
By contrast, if you think about the options that are left to Casey by the local conditions eliminating both large red and small blue shirts, there are only five remaining possibilities:
Red medium
Blue medium
Yellow small, medium, large
If we try to take away answer choice (D), which tells us that Casey must buy either a medium red shirt or a medium blue shirt, then Casey would have to choose all three shirts from the final three possibilities:
Small yellow
Medium yellow
Large yellow
But since the rule you pointed out makes it impossible to buy both a small shirt and a large shirt, these remaining possibilities could only account for, at most, two of the three shirts (since we would have to take out either the small yellow shirt or the large yellow shirt, even for a global question). Because of that third rule, Casey will always have to buy at least one shirt that is either red or blue.
Thus, we know that under the conditions of question 11, Casey will have to buy either a red medium shirt or a blue medium shirt, since only the medium size is now available for either of those colors--which means that (D) has to happen.
I hope that clarifies things!
Laura
