Thanks agps. That's what I thought the answer should be too but the answer was B. I couldn't figure out why.

agps wrote:answer is C.

from the question
let m be Mary's annual salary, j Jim's and k Kate's.
m-j=2(m-k)
m>j and m>k

from 1) j=30k, can't figure out m or k, insufficient
from 2)k=40k, can't figure out m or j, insufficient

together
we know that m>k>j
m-30=2(m-40)
m-30=2m-80
m=50
so the mean is 50+40+30/3 = 120/3 = 40, sufficient.

P.S. - you don't have to actually calculate the result, only understand that from 1 and 2 you can get m and that makes it sufficient.

Acording to Q

M - J = 2(M - K)
i.e 2k -J = M
we can use this value of M to find the avg

Now we know M =2K -J i.e. to find the avg we can substitute the value of M as 2K -J, so now we have two variables i.e J & K now conventionally we would require 2 equations to solve for 2 varibles, but this is not always true

here to find the avg we need sals of M,J,K

now M = 2K- J
sal of Jim =J
sal of Kate =K

we need to add these 3 and divide by 3 to get the avg

so lets us add these three
i.e. 2K - J + J + K
i.e 3K notice J gets Cancelled out so avg is 3k/3 =k

so avg is nothing but K's salary stnt B gives us that.

Hence ans should be B

good point samir. I have to be more caerefull, DS is always a trap if we don't look caerfully

I also think like you do, so i always try to simplify the question stem as much as possible before looking at the statements.