annual salary of the 3 people (GMAT PREP 1)

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In a certain year, the difference between Mary's and Jim's annual salaries was twice the difference betwen Mary's and Kate's annual salaries. If Mary's annual salary was the highest of the 3 people, what was the average (arithmetic mean) annual salary of the 3 people last year?

1. Jim's annual salary was $30,000 that year
2. Kate's annual salary was $40,000 that year.

The OA is B, but I got C. Could someone please explain. Thanks!
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by niketdoshi123 » Sat Aug 04, 2012 3:22 am
alex.gellatly wrote:In a certain year, the difference between Mary's and Jim's annual salaries was twice the difference betwen Mary's and Kate's annual salaries. If Mary's annual salary was the highest of the 3 people, what was the average (arithmetic mean) annual salary of the 3 people last year?

1. Jim's annual salary was $30,000 that year
2. Kate's annual salary was $40,000 that year.
Let Mary's salary be M
Kate's salary be K
and Jim's salary be J

According to the question stem

M - J = 2(M - K)
=> M = 2K - J
=> 2K = M + J

we need to find (M + J + K)/3
=> ( 2K + K )/3
=> K
Hence the question asks what is the value of K

and statement 2 provides the value of K
Hence sufficient