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Oil tank capacity

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Oil tank capacity

by GmatGreen » Mon Feb 24, 2014 2:31 pm
When 200 gallons of oil were removed from a tank, the volume of oil left in the tank was 3/7 of the tank's capacity. What was the tank's capacity?

(1) Before the 20 gallons were removed, the volume of oil in the tank was 1/2 of the tank's capacity

(2) After the 200 gallons were removed, the volume of oil left in the tank was 1,600 gallons less than the tank's capacity
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by Patrick_GMATFix » Mon Feb 24, 2014 2:47 pm
Note that there are only two unknown values: the capacity and the starting volume of oil in the tank (we know that ending volume is 200 less than starting volume). The prompt info gives us a simple relationship between these variables (a two-variable linear equation). Any data that gives us a different simple relationship (a 2nd linear equation) will be sufficient since we will have as many distinct linear equations as we have variables. For this reason, each statement is sufficient; no need to even do the work.

The full solution below is taken from the GMATFix App.

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by [email protected] » Mon Feb 24, 2014 5:27 pm
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

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