Self-study
All of the cargo ships of the Blue Star Line are over 100 meters long, and all of its passenger ships are under 100 meters long. ████ ██ ███ █████ ██ ███ ████ ████ ████ ████ █████ ██████ █████ ███ ██ ███ █████████ ███ █████ █████ ██ ███ ████ ████ ████ ████ █████ █████ █████ ███ ███ ███ █████ ███ ██████ █████ ███ ████████ ██████████ ██ ████ ████████ █████ ██ ████ ████ ██ █████ ██ █████ ███ ██████ ███ ███████████ ████ █████ █████ ████ ███ ████ ████ ███ ██████ █████ ███ ████ █████ ██ █ █████ ████ ████ ██ █████████ ██████ ██ ████ ████████
Stimulus Summary
We're given several descriptions (you can think of them as "rules") about two lines of ships, and a port served by them. The Blue Star Line's cargo ships are all over 100 meters long, its passenger ships are all under 100 meters long, and most of its ships were built before 1980. The Gold Star Line's ships, whether passenger or cargo, are all under 100 meters long and all built after 1980. Port Tropica is open only to these two lines of ships, and can only accommodate ships less than 100 meters long. A cargo ship named the S.S. Coral is docked at Port Tropica.
Analysis: Must Be True
You might be wondering where to start with a list of (mostly) conditionals like this. It would be counterproductive to try to diagram everything here, so it's better to try to draw inferences as you read. Notice that the rules start out more general — the details for the two shipping lines — and then are applied to a specific situation: Port Tropica, and the one specific ship docked there. So a good way to proceed as you read this is just to ask if we know anything else about the more specific situation (Port Tropica and S.S. Coral), given the general rules provided first (the details of the Blue Star and Gold Star Lines).
In other words, the minute we read that Port Tropica is open only to the Blue and Gold Star lines, and only accommodates ships less than 100 meters long, we know it must only accommodate the Blue Star Line's passenger ships, because all the Blue Star Line's cargo ships are above 100 meters long. The port can accommodate both the Gold Star Line's cargo ships and its passenger ships. And since we know the S.S. Coral is a cargo ship docked at Port Tropica, we know it must belong to the Gold Star Line. These inferences are probably a good place to start — we can now go to the answer choices and proceed by elimination.
18.
If the statements above are █████ █████ ███ ██ ███ █████████ ████ ██ ████ ██ ███ █████ ██ █████
a
The S.S. Coral ███ █████ █████ █████
Correct. Remember that in our analysis, we determined that the S.S. Coral must belong to the Gold Star Line. This answer choice makes one further inference: since all the Gold Star Line's ships were built after 1980, the S.S. Coral must have been built after 1980.
b
The S.S. Coral ███████ ██ ███ ████ ████ █████
Incorrect. In our analysis, we determined that the S.S. Coral must belong to the Gold Star Line. It can't belong to the Blue Star Line, because all the Blue Star Line's cargo ships are over 100 meters long, and Port Tropica (where the S.S. Coral is docked) can't accommodate ships over 100 meters.
c
Port Tropica is ██████ ████ ██ █████ ██████
This doesn't have to be true. The only details we are given about Port Tropica are that it is open only to Blue Star Line and Gold Star Line ships and only accommodates ships under 100 meters long. This rules out Blue Star Line cargo ships. We also know that one cargo ship, the S.S. Coral, is docked there and must belong to the Gold Star Line. But we don't know anything about whether Port Tropica permits passenger ships or not. The fact that the only ship we know about that is docked at Port Tropica is a cargo ship doesn't necessarily mean only cargo ships can dock there.
d
Port Tropica is ███ ██████ ██ █████ ██ ███ ████ ████ █████
This doesn't have to be true. We know that Port Tropica is not served by the Blue Star Line's cargo ships, because they are all over 100 meters long. But it could still be served by the Blue Star Line's passenger ships. If you picked this answer choice, you might have carried over the assumption from (C) that Port Tropica isn't served by passenger ships, which would (if it were true) completely exclude the Blue Star Line from Port Tropica. But remember that that assumption doesn't come from the stimulus. The only ship we know about that is docked at Port Tropica is a cargo ship, but this doesn't mean passenger ships are excluded from docking at the port — and if it's possible that Port Tropica is served by passenger ships, then it's possible that Port Tropica is served by the Blue Star Line.
e
All of the █████ ██ ███ ████ ████ ████ ███ █████ ████ ███ ██ ███ █████ ██ ███ ████ ████ █████
This doesn't have to be true. We only know that most of the Blue Star Line's ships were built before 1980. This leaves open the possibility that some of the Blue Star Line's ships were built at some unspecified date after 1980. So even though all the Gold Star Line's ships were also built sometime after 1980, it's possible some of the Blue Star Line's ships were built at the same time or later than the newest of the Gold Star Line's ships. This would mean not all the Blue Star Line's ships were older than any of the Gold Star Line's ships.