So the grove is split two ways into Northern half-Southern half and Western half-Eastern half. Think of a circle split horizontally into 2 parts and also vertically into two parts.
Using both statements, we know that there are 18 trees in the northern half so there must be 18 in the southern half. We also know that there are 18 trees in the eastern half and so there must be 18 in the western half.
For ease, let's assume each quadrant has 9 trees.


The average yield in the northern half is 60 and average yield in the eastern half is 55. Note that 9 trees (in red) overlap in northern and eastern halves. We have no idea about the yield of the south west quadrant. Hence this is certainly not sufficient. Also, we don't know how many trees actually overlap and the yield of those trees.

Answer (E)
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I also go with E. the south west corner is missing

can someone explain why? and if the southwest corner was given how would we have calculated the yield.

The correct answer is E. Check here: http://gmatclub.com/forum/if-a-certain- ... l#p1279526
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