Actually I got this one. This is how I solved it.

M - mary's annual salary
J - Jim annual salary
K - Kate annual salary

Its given M-J = 2(M-K) -----> I .
From this we can conclude that J < K.
So Mary has highest sal, K second highest and then J.

From 1) J = 30,000

Assuming Mary sal to be 50000 and plugging it in I (we can assume mary salary as M is on both sides of the equation.

(50,000 - 30000) = 2(50000 - 40,000) average is 5+3+4/3 (taking out 0's)

Assume Mary salary = 40,000
(40,000 - 30,000) = 2 (50,000 - 45000) average is 4+3+4.5/3

Since both the values are different we can cancel AD.

From B,
Kate annual salary is 40,000

So again assuming M salary we get
(50,000 - 30000) = 2(50000 - 40,000) average is 5+3+4/3 (taking out 0's)

Assuming M to be 70,000
(70,000 - 10,000) = 2(70,000 - 40,000) average is 7 + 1 + 4/3

Consider any value of M which satisfies Eqn 1 and the average is always the same. Therefore the answer is B.

Thanks.