Since à is less than 70% of numbers in L then a =0.7*200 = less than 140

And b is less than 30% of number less L then b =0.3*200 = less than 30

Since b-a can assume any number btw 0 to 140
Hence the answer is D

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Carcass, can you please help with the interpretation of the question?
List L consists of 200 numbers that are between 0 and 40. Does it mean that numbers in the list are between 0 and 40? Where 0 and 40 comes into play?

This question is really tricky

First of all, L is a set of 200 numbers between 0 and 40.

Now the numbers could be 1,2,24,39 but also 1/2,7/8,39.84213474724475444.................and so on and the questions does not mention that they are integers but only they are not equal each other

A is less than 70% of the numbers in set L. 70% of 40 is 28 and we left with 12 numbers

We do not know

1) the distribution of the numbers
2) people could think that a is < to 70% of the numbers in L and we could think that the 70 is from 40 to 0 down but also a could be positioned between 1 and 6 or 70% of the number could be compressed in the range between 38 and 40 for example from 38.5 to 40 and a is 38.2 for example
3) The same reasoning could be applied to b

So we do not have enough information about.

D is the answer

Carcass, what skills , understanding of relevant concepts are required to address problems of this type. What are examples of similiar problems?

Good question

Well, on the GRE many questions are conceived to be at the intersection of several notion/skills.

We do have questions that for example are pure abolsute value or geometry but many others that test you in more than one area

In this case is number properties but also overlapping sets somehow.

In the end, that's why the GRE is a test of logic.

You should be flexible, applying all the notions you have incorporated and studied for this exam