hazara wrote:GauchoMarx wrote:The stimulus does not mention anything regarding the state of suspension bridges and whether or not they need rehabilitation, all it says is that at least some of the bridges that are in need of rehabilitation are not suspension bridges and that of the bridges that were built according to faulty engineering design, none of them are suspension bridges.
This is a must be true question and it's not the case that A must follow from the stimulus. Even though none of the bridges built with faulty engineering design were suspension business, it could be the case that some of the other bridges built between 1950 and 1960 are suspension bridges.
(C) must follow from the stimulus. Because all bridges built between '50 and '60 are in need of rehab, and because some bridges built in this period were built on faulty design, then some bridges built on faulty design are in need of rehab.
doesnt that basically tell us that there are atleast somesome suspenstion bridges that are not in need of rehabilitation?
I think you
may be trying to take the contrapositive of the 'some' statement. If you are, you need to remember that 'some' statements are
reversible, but we cannot contrapose them the way you do conditionals. From the stimulus, we know that:
- some [serious rehab bridges] are [non-suspension bridges]
From this we CAN infer that:
- some [non-suspension bridges] are [serious rehab bridges]
If you were going to attempt to make a sort of 'contrapositive', it might have looked like this:
- some [suspension bridges] are [NON-serious rehab bridges]
But that's not legal - you cannot reverse and negate 'some' statements. You are
only allowed to reverse them -
they do not behave like conditionals.
To illustrate why this is true, imagine a world where all the apples are red - every last one of them. Now, there are also red cars, and shoes, and hats, but the important thing is that ALL the apples are red. So, from this last part, we know that:
- some [red things] are [not apples]
Now, if we try to make a sort of 'contrapositive' of that, we'd get:
- some [apples] are [NOT red]
But wait a second! I just told you at the beginning that in this world ALL the apples were red!! You can't now say that some apples AREN'T red - that clearly can't be true. This inference is illegal.
In the example in the question, what if it were true that there are only 3 suspension bridges in the entire world, and all of them were build between 1950-1960. There's nothing in the stimulus that prevents that! So, then we would know that all 3 of those suspension bridges were in serious need of rehabilitation. And since they are the only suspension bridges in existence, that would mean that ALL the suspension bridges in the world were in serious need of rehabilitation! We don't know if this situation is true, of course, but nothing in the stimulus prevents it - so it's possible. Thus, it does not
have to be true that some suspension bridges are NOT in serious need of rehab!
The important mechanical habits to remember about 'some' statments are that:
- 'some' statements are reversible
'some' statements cannot be contraposed like conditionals
'some' statements may mean 'all'
'some' statements are not linkable to one another