A certain airline’s fleet consisted of 60 type A planes at the beginning of 1980 …

Correct answer is (D)

This problem is extremely easy to solve if you leverage the answers by backsolving, but it can be confusing and time-consuming if you try to create an equation or inequality. Since it will be clear on this question if you need a smaller or larger number after your first try at backsolving, start in the middle with (C). In 8 years, there will be 60-8(3) or 36 type A planes and (8)(4) or 32 type B planes. No: B is not greater than A, so you need a larger number. In 9 years, there will be 60 – (9)(3) or 33 type A planes and (9)(4) or 36 type B planes. Jackpot: 9 is the first answer that makes B > A so (D) is correct. You can literally sit back in your chair and do this with very little mental effort in under one minute with backsolving. If you solved this problem in any other way, you simply did not choose the best approach (and there is a good chance you got it wrong or spent 3+ minutes). In other algebraic interpretation questions, it might be better to use algebra—backsolving will be either impractical or impossible—but not so on this question!
 
In working with students, I use this question to see if they are truly leveraging the full toolbox to solve GMAT quant questions. If you don’t see the easy backsolving approach here, it means you are not looking for it. There are numerous ways to solve GMAT problem solving questions: you can use algebra or other “traditional” math approaches; you can pick numbers to remove abstraction in percent or variable word problems; you can use conceptual thinking or logic; you can actively leverage the answer choices (backsolving). 
 
When GMAT math questions are created, they are designed to punish people who always approach questions the same way. A typical 700+ level question on the exam will make you use fairly hard algebra to go most of the way to the answer, but then the only way to get it correct at the end is to leverage the answers. The problem can’t be solved without answer choices to leverage, yet the “algebra machine” student will knock his or her head against the computer trying to solve the problem in a vacuum without actively using the answers. More commonly, questions are created in which one approach is extremely difficult or time consuming, while another approach is quite simple (as in this question). 
 
Before you jump headfirst into a problem, always take the time to consider what approach will work best. You will not always make the right choice initially, but your instincts will become better and better as you complete more GMAT official practice problems. Remember: You can’t be a one-trick pony on this test if you want to score highly!