Hi GmatGreen,

This DS question is built around a pattern worth knowing: "one variable, one equation....I CAN SOLVE IT."

The prompt tells us that after 200 gallons of oil are removed from a tank, the volume of what's LEFT = 3/7 of the total tank's capacity. The question ask for the total capacity...

The key to this question is the relationship between "partially full" and "full" in this tank. We don't know if the tank was full to begin with, so we don't know what fraction of the total capacity the 200 gallons represents.

Fact 1: Before the 200 gallons were removed (I assume the "20" that's listed is a typo), the oil in the tank was 1/2 the tank's capacity.

We can translate this info into an equation...

(Full)(1/2) - 200 = (Full)(3/7)

This is one variable and one equation, so we CAN solve for the capacity of the tank.
Fact 1 is SUFFICIENT

Fact 2: After 200 gallons were removed, the volume left was 1600 gallons less than the total capacity.

Again, we can translate into an equation....

(Full)(3/7) = (Full) - 1600)

This is also one variable and one equation, so we CAN solve for the capacity of the tank.
Fact 2 is SUFFICIENT

Final Answer: D

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