A certain law firm consists of 4 senior partners

[RonPurewal]

the problem stated that 2 groups are considered different if at least one group member is different.

why didn't we use permutation instead of combination?

That has nothing to do with whether order matters (= the difference between those two concepts).

In this statement, all they're doing is providing a definition of the word "different".
Yes, seriously—they're just telling you what "different" means. They're just saying that a group consisting of A and B is, indeed, "different" from a group consisting of, say, A and C.

I can't imagine that anyone really needs to be told this (i.e., I don't think anyone thinks that the word "different" is reserved for groups with nothing in common). But they're just bending over backward to make sure that the problems are unambiguous.

Basically, the GMAT is never "tricky"——but, sometimes, they try so hard NOT to be "tricky" that the resulting sentences can accidentally become confusing ("Nah, they wouldn't have to tell me that; this must mean something else").

[njain988]

Hi Ron ,

the question says : 2 groups are different if atleast one member is different .
Doesnt that rule out the possibilty of 3 seniors??

Also , how do you approach this problem if it was NOT explicitly stated that "2 groups are different"??

[RonPurewal]

see, here, they are trying so hard to AVOID ambiguity that, ironically, the problem is harder to read as a result. this is just a statement of the obvious.

basically, they are just saying this:
the group "A, B, C" is 'different' from the group "A, B, D".

in other words, they're just saying that two groups are not 'the same' unless EVERYONE is the same.

of course, these are the same definitions of 'same' and 'different' that we have all used since we were 6 years old. so, basically, you can just ignore this part.

[RonPurewal]

along the same lines—whenever you divide by some quantity, they will always tell you that the quantity is non-zero (even though the rules already say this).
and so on.