Step 1: Understand (Glance Read Jot)
The answers are spread out, which suggests Estimation. Choices (A) and (B) are close to 100, choice (C) is close to 1,000, and (D) and (E) are in the ballpark of 10,000. The answer choices are also clean values, so consider Working Backwards.
The question gives numbers in terms of millions and billions and in terms of pounds and gallons; this question might be testing Translations, namely unit conversions.
The words how many more indicate that you are looking for the difference between two numbers (so, subtraction), which means that working backwards might be a bit more difficult; it is easier to work backwards when the answers are a simple value to plug in, rather than a relationship. The word approximately indicates once again that estimation might be possible.
Jot down the given numbers, conversion rates, and what the question is asking for:
980 million pounds → 2.7 billion pounds
1 billion = 109
1 million = 106
1 gallon = 8.6 pounds
How many more gallons?
Step 2: Plan (Reflect Organize)
The answer choices represent the difference between the 980 million pounds in 2007 and the 2.7 billion pounds in 2014. To work backwards from the answer choices, all values (2007, 2014, and the answer choices) will need to be converted to the same units.
The likeliest mistakes are decimal misplacement (wrong number of zeros) and multiplicative inverse errors (dividing by 8.6 versus multiplying by 8.6). Be careful with units (pounds, gallons) and with powers of 10. When a GMAT problem underlines a word, pay attention: make sure the answer is in millions of gallons.
Among the choices, the easiest numbers to work with are 100 and 200, and it’s generally recommended to work backwards from (B) or (D), so work backwards from (B) first, then evaluate whether to continue working backwards or to estimate.
Because the answer choices are so spread out, consider using estimation throughout to make the computation easier.
Step 3: Solve (Do the Work)
Working Backwards
First find the difference between production in 2007 and 2014. Since 980 million is approximately 1 billion, the difference is approximately 1.7 billion, or 1700 million pounds.
Be careful! Trap answer (C) is 1700, but the answers are in millions of gallons, not millions of pounds. The correct answer will be the one that, when converted to pounds, equals 1700.
To convert gallons to pounds, you would multiply each answer choice by 8.6. However, simplify the computation by benchmarking.
Choice (B):
200 million gallons @ 8 pounds/gallon = 1600 million pounds of milk
200 million gallons @ 9 pounds/gallon = 1800 million pounds of milk
Since the actual weight is 8.6 pounds/gallon, 200 million gallons will be about halfway between 1600 and 1800 million pounds of milk. The target difference is 1700 million pounds; this is the correct answer.
Algebra
See
the Official Guide solution for the algebraic method of converting the units and solving.
The correct answer is (B).
Hit Kudos, if it is useful.