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There isn't really any algebra to this one.
The question tells us that the burn rate for a given train is directly proportional to its speed, i.e a 10% increase in speed causes a 10% increase in coal consumed. In order to figure out how much coal the train will consume on this particular 60 mile trip, we need two pieces of information: its speed and a point of reference from which to calculate the burn rate.
Statement 1 says that on a previous trip (which was also 60 miles) the trained burned 100 pounds of coal at a speed of 60 MPH. We now have a starting point for burn rate, but do we know how fast the train will be going on the current trip? Nope, so Statement 1 is not sufficient on its own.
Statement 2 tells us that the train is going 30 MPH on this particular trip. That's one of the pieces of information we needed, but we have no way of calculating the burn rate. Statement 2 is insufficient.
When we put them together, we can see that the train is traveling the same distance at half the speed of the trip in Statement 1. Since speed and burn rate are directly proportional, we know that a 50% decrease in speed will lead to a 50% decrease in coal burned. Thus, we know that the train will burn 30 pounds of coal on this trip. Answer C is correct.
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