- Mon May 25, 2020 12:44 am
Hi jm123! You did a great job of figuring out why (B) is wrong.
(B) allows for the possibility that 2 and 4 could go on the same day, which means that 4 could go on Wednesday. Similarly, it allows for the possibility that 8 and 4 could go on the same day, which means that 4 could go on Saturday. So (B) does not do a good job of substituting in for our rule. (Remember, we're looking for an answer that successfully constrains 4 to Thursday and Friday).
Now let's look at (A). (A) restricts 4 to being on the same day as, or the day before/after, 7. Where can 7 go again? Well, only Thursday or Friday, since it has to come after 3 but before 5 (because of Rules 3 and 4). Let's look at both of those scenarios.
So if 7 is on Thursday, that means 4 would have to go on the same day (Thursday), or the day before/after (so Wednesday or Friday). No problem with 4 going on Thursday or Friday, but if it can go on Wednesday then (A) is not a good answer. Luckily, it can't. Why not? Well, if 7 is on Thursday, then that means 3 has to go on Wednesday (because of Rule 3). And because of Rule 1, 4 can't go on the same day as 3, which means Wednesday's out. So that only leaves the possibility of 4 going on Thursday or Friday.
What about if 7 is on Friday? Well, that means 4 would have to go on the same day (Friday), or the day before/after (Thursday or Saturday). No issues with 4 going on Thursday or Friday. But if it could go on Saturday that would be a problem! It can't though. If 7 is on Friday, that means 5 has to be on Saturday (because of Rule 4). Because of Rule 1, 4 can't go on the same day as 5, so Saturday's not an option. Which leaves only Thursday or Friday for 4.
So (A) only allows for 4 to go on Thursday or Friday, which makes it a great substitute for our rule and a great answer.
Hope that helps!