Hi,

I tend to suffer from “choosing the wrong strategy at the wrong questions” when I need to choose between plugging in numbers and solving the questions using algebra.

I must admit that I prefer solving questions which is not very efficient at the GMAT… but sometimes (for example – please find below a question for which plugging in numbers in not an efficient strategy, as also mentioned in the solution)

Could you please advise where is it clever to use each strategy?

Thanks
Liron


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A publishing company produced a record high of 180 books today. In the 9 days prior to today, the same publishing company had produced an average (arithmetic mean) of x books per day. If today’s record high increased average daily production to y books per day, what is x in terms of y?

- (9y+180)/10
- (10y+180)/9
- (9y-180)/9
- (10y-180)/9
- 10y/9-180

Because this is a Variable In Answer Choices (VIC) problem, we can also use a Picking Numbers and Calculate a Target approach. However, for this problem, the Picking Numbers method turns out to be cumbersome and time-consuming; it is more efficient to tackle it using Direct Algebra.

9 days prior to today,
avg = x books per day
total no of books = 9x


now,
avg = y books per day
tot no of books = 10y
no of books produced today = 180


(9x+180)/10 = y
9x+180 = 10y
x = (10y-180)/9

Option D

I actually disagree that picking a number is so difficult. It can require you to do some math calculations, but I think it's a good thing to be sharp at such calculations.

In this problem, I want to pick a number of books to be the average of the prior 9 days. I am going to say x=20, so there were 180 total books over the last 9 days. 180 added to the 180 on the tenth day gives me 360 books over 10 days, which averages to 36 books. Thus y=36.

Okay, so x=20 and y=36. Question is what is x (mentally ignore the "in terms of" on any VICs question). x=20, so that is our target number.

At this point it's just a matter of cranking through the answer choices using y=36 to find 20. I would quickly eliminate:

A--I can tell that I will not get an integer when I divide the top by 10.
E--will give me a negative number.

Those are the easy two; let's actually calculate the rest.
B--gives me 360 +180 = 540. Dividing that by 9 will give me 60, not 20.
C--I can look at that and know it will be too small.
D--bingo! There's the winner.

_________________
Jamie Nelson
ManhattanGMAT Instructor

Also, Ron has some really good tips on when to pick numbers in this thread: vics-chapter-7-t2411.html

Please let us know if you have further questions!

_________________
Jamie Nelson
ManhattanGMAT Instructor