Most train inferences

[HarmonRabb]

Hi, 3 part question, I'm hoping for some guidance on.

Part 1:

I came across a formal logic practice question riddled with somes and mosts on some blog and I was surprised to see they said inferences could be made. I was highly dubious when I saw all the somes and mosts and not a single all.

This is the passage:

There is some debate as to whether great-crested grebes are common in Swansea, but it is known that most grebes live in North Savoy and some grebes live within a mile of a lake. Also, most grebes inhabit boggy areas.

Choose the inference that can be properly made from the premises:

a. Some grebes in Swansea live within a mile of a lake.

b. Some grebes in North Savoy live within a mile of a lake.

c. Some grebes in North Savoy inhabit boggy areas.

The answer provided:

Correct answer: c. This can be a tricky one. The tough part is see that when most members of a group have some characteristic, and most members of the same group also share another characteristic, then there has to be some overlap: at least one grebe is going to have to live in North Savoy and inhabit a boggy area.

Part 2

How would you diagram this? I know arrows don't have to only go horizontally, but my attempted looked nothing like I've come across in PowerScore before, every relationship coming off of "grebes" so nothing chained together.

Part 3:
(Sorry for the long question). I know that only time you can make an inference from two most relationships is A<---m---B---m--->C, but can you give an example of LSAT level text that would yield this relationship? I'm having trouble picturing it past a super simple example.

Thank you very much

[Dana D]

Hye HarmonRabb,

This question actually is an example of the A<---m---B---m--->C relationship you described. Remember that on the LSAT, "most" means 51% or more. So, in this case, more than half of the grebes live in North Savoy, and more than half live in boggy areas. That means that at least one ("some") live both in NS and in a bog. There is no way to account for more than half the population in both conditional statements without having some overlap.

If there are 100 grebes and most live in North Savoy, that means at least 51/100 live in NS. Additionally, we are told most live in a boggy area - again this means at least 51/100 live in a boggy area. There must be at least one grebe that both lives in NS And in a bog, because at least 51/100 of the grebes we are talking about fit into both categories. Does that make sense?

[HarmonRabb]

That's much more clear now, thanks very much!