If a certain grove consists of 36 pecan trees, what was the yield per tree last year?
(1) The yield per tree for the 18 trees in the northern half of the grove was 60 kilograms last year.
(2) The yield per tree for the 18 trees in the eastern half of the grove was 55 kilograms last year.

Sanjeev's explanation is perfect. I thought I'd add some images for any students who may prefer a visual explanation.

GOAL: Determine the yield for the ENTIRE grove.

Statement 1 tells us about the shaded trees shown here.

Since there are trees we know nothing about, statement 1 is not sufficient.

Statement 2 tells us about the shaded trees shown here.

Since there are trees we know nothing about, statement 2 is not sufficient.

The statements COMBINED could look something like this.

Since it's still possible that we have no information about some trees, we can't answer the target question with any certainty.

Answer: E

Cheers,
Brent