Hi All,

We're told that on a map, 1/2 inch represents 100 miles. We're asked - according to this map - how many miles is City X from City Y. This question is based around the idea of 'unit conversions' (which is really just basic Arithmetic. The prompt gives us the ratio of map-inches to miles (re: 1/2 inch = 100 miles, so 1 inch = 200 miles). Thus, to answer this question, we need to know how many inches apart the two cities are.

(1) City X is 3 inches from City Y on the map.

Fact 1 gives us exactly what we need to know to answer the question. 3 inches = 600 miles, so the cities are 600 miles apart.
Fact 1 is SUFFICIENT

(2) Cities X and Y are each 300 miles from City Z.

The information in Fact 2 essentially creates a circle with City Z in the center and City X and City Y a 'radius' of 300 miles away. As such, the distance between City X and City Y could as far as 600 miles (if they're on exact opposite radii from City Z) or right next to one another.
Fact 2 is INSUFFICIENT

Final Answer: A

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