#10 - All bridges built from 1950 to 1960 are in serious

[ksandberg]

Hello,

I am having a hard time understanding this question. I do not even know where to begin. Can you please tell me why c is correct?

Thank you.

[Nikki Siclunov]

Hi,

Thanks for your question!

The stimulus uses deductive logic, combining elements of Conditional Reasoning and Formal Logic. If you're a student in one of our FL LSAT courses, the concept of Formal Logic is explained in a Virtual Module under Lesson 8. If you're using the Logical Reasoning Bible, there is a chapter dedicated to this topic as well. Thankfully, while Conditional Reasoning is still important in both LG and LR, Formal Logic is becoming increasingly rare on the test, so your difficulty with this question should not be a cause for alarm

That said, let's break down the stimulus:

All bridges built from 1950 to 1960 are in serious need of rehabilitation. Some bridges constructed in this period, however, were built according to faulty engineering design. That is the bad news. The good news is that at least some bridges in serious need of rehabilitation are not suspension bridges, since no suspension bridges are among the bridges that were built according to faulty engineering design.

• 1. Bridges built 1950-1960 Need of rehab
  
  2. Bridges built 1950-1960 Faulty design
  
  3. Need of rehab Suspension
  (I'm excluding this statement from the diagram, as it's an inference the author is drawing from combing claims #1, 2, and 4)
  
  4. Suspension Faulty design

Applying the rules of Formal Logic, the propositions above can be combined into the following conditional chain:

Suspension Faulty design Bridges built 1950-1960 Need of rehab

Now, let's consider the possible inferences that can be drawn from this chain:

1. If some bridges built between 1950-1960 have faulty design, but no bridge that is built according to a faulty design is a suspension bridge, we can conclude that some bridges built between 1950-1960 are not suspension bridges:

• Bridges built 1950-1960 Suspension

2. Since all bridges built between 1950-160 are in need of rehab, and some are based on faulty design, we can conclude that at least some bridges that are based on faulty design are in need of rehab:

• Faulty design Need of rehab

The correct answer choice needs to conform to either of these two inferences. There is, of course, one more inference that we can draw from these relationships. However, since it was already included in the stimulus, we are unlikely to be tested on it:

• Need of rehab Suspension

Answer choices (A) and (B) are incorrect, because no conclusion can be drawn about suspension bridges. All we know is what must be true of non-suspension bridges.

Answer choice (C) is consistent with the second inference (above), and is therefore correct.

Answer choice (D) must be false, since all such bridges are in need of rehab.

Answer choice (E) is also false, because not a single bridge that is built on faulty design is a suspension bridge.

Hope this helps!

[MikeJones]

Hi,

4. Suspension Faulty design[/list]

I don't think this is a biconditional.

[James Finch]

Hi Mike,

Correct, this is not a biconditional. According to the stimulus, a bridge cannot be both a suspension bridge and have the faulty engineering design (although it could still be neither.) We are given:

Faulty engineering design suspension bridge

Which means that

Suspension bridge faulty engineering design

so

Faulty design Suspension Bridge Bridges built 1950-1960 Need of rehab

is the logical chain that we get from the stimulus, including the inferences that some bridges built between 1950 and 1960 aren't suspension bridges and that some of the bridges with faulty design are in need of rehabilitation.

[MikeJones]

Hi Mike,

It is a biconditional, as there is no possible overlap between the two groups. We are given:

Faulty engineering design suspension bridge

Which means that

Suspension bridge faulty engineering design

so

Faulty engineering design Suspension bridge

because, according to the stimulus, a bridge both a suspension bridge and have the faulty engineering design (although it could still be neither.)

Hope this clears things up!

Kind of confused. It looks like you may have made a typo somewhere. I was under the impression that biconditionals can only occur when an either or statement is combined with a but not both statement.

Those two statements just seem to be contrapositives of each other. For it to be a biconditional, I thought it would have to be:

Faulty engineering design --> /Suspension Bridge
/Suspension Bridge --> Faulty engineering design
=
Faulty engineering design /Suspension Bridge, which seems the same as A B

Am I wrong in my understanding here? It would appear that a statement like A-->/B is just the same as A B if you're right, so why wouldn't we just write it out like that in the first place?

[James Finch]

Hi Mike,

Good catch, you are correct! I've edited my answer to reflect this. As it is possible with the setup that the stimulus gives us does not rule out the possibility that some of the bridges that lack faulty designs are not suspension bridges, and contrapositively that if a bridge is not a suspension bridge, it may not have a faulty design either. For the purposes of the question, this means that it is possible that none of the bridges needing rehabilitation are suspension bridges, and that none of the bridges built between 1950 and 1960 are suspension bridges.

[bbjigglercakes]

I am so confused as to why A) would not be the right answer?

the stimulus says: some bridges in serious need of rehab are NOT suspension bridges.

and A) some suspension bridges are not in serious need of rehab

I honestly dont know what the mistake is btw the two?

i saw someone mention only not suspension bridges are mentioned in the stimulus nothing about actual suspension bridges needing rehab.

in the stimulus doesnt some bridges that need rehab are not suspension bridges imply some suspension bridges don't need rehab?

or is it:

some NOT suspension bridges need rehabilitation ---> i feel like this might be the correct answer if A was phrased this way.....

[Robert Carroll]

bbjigglercakes,

Statements involving the word "some" are reversible, but answer choice (A) is not reversing the "some" statement. It's trying to contrapose it. But "some" type statements do not have contrapositives.

An example: imagine a family receives 5 gifts for Christmas, 3 of them toys and 2 of them other types of electronic device (let's say, calculator and a wireless speaker). Let's say that each of the 5 gifts requires batteries to operate. Then we could say "Some of the things that require batteries are not toys," which is true, because the calculator and wireless speaker require batteries, but aren't toys. We cannot say, though, that "Some of the toys do not require batteries," because that's false in my situation - all the toys, like anything, require batteries.

So:

require batteries toys

is true, while

require batteries toys

is false. So these statements aren't identical.

Robert Carroll