Andre_p wrote:

Can you give me an example of the equations that would be setup for both?

Well, we can always setup equations, but to solve DS problems, you don't really need to do that. All that is needed is, whether we can solve the problem.

In a refinery, the capacity of oil tank A is 70 percent of the capacity of oil tank B. How many more gallons of oil are in tank A than in tank B?

1) Tank A is 90 percent full; tank B is 50 percent full

2) When full, tank A contains 50,000 gallons of oil
In this questions, what is already given is that -

A = 0.7 B - (1) - A and B are capacities of tank A and B respectively.

The question asks for the difference in Oil in gallons in tank A and B. So what we need to know is the amount of oil in both Tank A as well as B.

So the above question can be reduced to - How much oil is there in tank A and B?

Note that tanks may not be full with the oil.

Let's look at the condition2 first since it looks easier.

2) tells us the capacity of A. So now we know the capacities of A as well as B from (1).

But this does not suffice, since the tankers may not be full.

Lets look at condition 1.

1) gives us the amount of oil in each tank as percentage of its capacity.

Condition 1, by itself doesn't tells us anything. Since we don't know the capacities of A and B.

From 2, we know the capacity of each tanker. If we combine 1 and 2, we know how much oil is there in each tanker. Voila! this is what we reduced the original question to.

Hence C.

HTH...