Linear Thinking and Data Sufficiency
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This article comes under the heading of GMAT Math, which is a misuse of the term. Very often, the GMAT isn’t about math at all, but rather about common sense – and the right approach.
Take a look at the following OG problem (OG 12th edition, p. 284, Q130):
If Juan had a doctor’s appointment on a certain day, was the appointment on a Wednesday?
(1) Exactly 60 hours before the appointment, it was Monday.
(2) The appointment was between 1:00 pm and 9:00 pm.
Go ahead and take 2 minutes to solve, then continue reading.
As you have probably surmised, there’s no need for fancy math here – it’s an everyday problem – a guy scheduling an appointment to see his physician. The catch lies elsewhere – in the form of linear thinking test takers fall pray to when trying to make a rush job of what seems a simple problem. When reading the question stem, most test takers have a vision of Juan entering his doctor’s office. Now, a doctor’s appointment usually takes place during the working hours, 9-to-5 part of the day. In our vision, the sun is shining on Juan as he enters the cool, air-conditioned waiting room. This sort of image is hard to shake, which is exactly what our friends at the GMAT are counting on: when the test taker then goes and reads stat. (1) (60 hours earlier, it was a Monday), linear thinking will make sure that the sunny image “transfers” to Monday as well. When we read stat. (1), we’re unconsciously thinking “Monday morning/afternoon.” If we then add 60 hours (twice 24 hours plus an extra 12 hours, or 2 1/2 days), we’ll find ourselves on Thursday evening, not Wednesday. So the answer to the question “was the appointment on Wednesday?” is ‘NO’, and what does that mean? Confusion combined with a ticking internal clock can cost a question because of sheer frustration. A clear head is the key to answer DS questions correctly, and such clarity requires the right approach.
Use this article to recall the right approach for DS yes/no questions. Upon reading the question stem, STOP (before moving on to the statements) and summarize the following:
This is a DS Yes/No question.
For the answer to be “yes,” the appointment must be on a Wednesday.
For the answer to be “no,” the appointment must be on another day (Tuesday, Thursday, any day BUT Wednesday).
Now, with this in mind, go and look at the statements, and try to show that they can lead to both a yes and a no, and thus are insufficient.
The significance of the 60 hours now becomes clear: If we begin the 2 1/2 day period on Monday at 01:00 AM, Juan will have the appointment at 13:00 on Wednesday afternoon (add two days to reach Wednesday 01:00 AM, then add half a day) and the answer is “yes.” But if we begin the 2 1/2 day period 12 hours later on Monday 01:00 PM, then the appointment will also occur 12 hours later – at 01:00 AM, Thursday, and the answer is “no.” Thus, statement(1) alone is insufficient.
When moving to stat. (2), watch out for linear thinking again – stat. (2) ALONE doesn’t tell you on WHICH DAY the appointment was scheduled. You need both statements to stand a chance to fix the schedule on Wednesday, and fix it they do: For the appointment to be at 1:00 PM, the 2 1/2 day period needs to start at 01:00 AM two days earlier. Likewise, for the appointment to be scheduled at 9 pm (eight hours later), the 2 1/2 day period needs to start at 9 AM two days earlier. If, according to stat. (1), that day is a Monday, then the appointment MUST be sometime between 1 PM and 9 PM on Wednesday, and the answer is a definite “yes,” or sufficient.
1 comment
Somsubhra Mukherjee on January 29th, 2012 at 9:54 am
Really sounding example.