Hi ankitaverma,

Sometimes DS questions don't give you a sense of what you need to know until you read the 2 Facts.

We're told that a grove has 36 trees in it and we're asked what the yield/tree was last year? This question implies that we'll be using the average formula.

Fact 1: The yield/tree for the 18 trees in the northern half of the grove was 60 kg.

This means that the TOTAL yield for the NORTH half was 18x60 = 1080kg. However, we don't know what the yield was for the SOUTH half of the grove.
Fact 1 is INSUFFICIENT.

Fact 2: The yield/tree for the 18 trees in the eastern half of the grove was 55 kg.

This means that the TOTAL yield for the EAST half was 18x55 = 990kg. However, we don't know what the yield was for the WEST half of the grove.
Fact 2 is INSUFFICIENT.

Combined, we know that the grove can be broken down into North & South and East and West, but we don't know how many trees are in each subcategory (how many are in the Northwest, for example). Without THAT specific information, there's no way to figure out the overall average.

Final Answer: E

GMAT assassins aren't born, they're made,
Rich

Rich'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