Hi gmat_winter,

While this question is not worded in proper GMAT "style", the logic behind it is simple enough.

We're told that an Orchard suffered damage (meaning 'lost crops') due to 2 things:
1) A hailstorm
2) Attacks by pests

The question asks for what PERCENT of the crop was lost due to the STORM.

Fact 1: TOTAL losses = 6 tonnes

This tells us nothing about the number of tonnes that were lost due to the storm, so we cannot answer the question.
Fact 1 is INSUFFICIENT

Fact 2: PESTS destroyed 10% of the crop

This tells us nothing about total losses or losses due to the storm.
Fact 2 is INSUFFICIENT

Combined, we know:
TOTAL losses = 6 tonnes
PESTS destroyed 10% of the crop

We know that 10% of the crop were destroyed by PESTS, but we don't know how big the TOTAL CROP was; thus, there's no way to gauge how many tonnes this 10% represents. Total LOSSES = 6 tonnes, but there's no way to link this information to the number of tonnes that were destroyed by PESTS, so we cannot determine how many tonnes were destroyed by the storm.
Combined, INSUFFICIENT.

Final Answer: E

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