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by Rich Cohen, [email protected] www.EMPOWERgmat.com
This series of emails and PMs focuses on situations that many Test Takers face during their studies. The names of the original posters have been changed to protect their identities.
The Basics Behind Prime Factorization
Hi Rich,
This question has kept me occupied for the entire day. I am trying to arrive at a general concept here.
"If a number Z is divisible by X, when would it also be divisible by Y"?
For Example: the number 108 is divisible by 27 and also divisible by 54. When would a number be divisible by both 27 and 54? What's the general concept hidden behind this problem that GMAT would try to test?
There is something it has got to do with the factors, but I just can't get clarity.
Uter
Hi Uter,
The concept that you've mentioned in this post is ultimately about "prime factorization" - by knowing what prime factors divide evenly into a number, you can figure out all of the possible values that can divide evenly into that number.
For example, the number 20 = 2x2x5
This means that the following numbers will divide evenly into 20:
1
2
4 = 2x2
5
10 = 2x5
20 = 2x2x5
This rule tends to come in handy when dealing with really big numbers or when trying to figure out the values at which numbers overlap with one another.
For example, what's the least common multiple of 4 and 6?
4 = 2x2
6 = 2x3
Notice how BOTH terms include one "2"? In this scenario, you DON'T count that 2 twice – you count it ONCE.
The LCM of 4 and 6 = 2x2x3 = 12
The Standard Work Formula Question That You Might See on Test Day
Dear Rich,
I know this is a work question, but I got stuck and I’m not sure how best to approach these questions.
If Sally can paint a house in 4 hours, and John can paint the same house in 6 hour, how long will it take for both of them to paint the house together?
A. 2 hours and 24 minutes
B. 3 hours and 12 minutes
C. 3 hours and 44 minutes
D. 4 hours and 10 minutes
E. 4 hours and 33 minutes
My workout :
1/4 +1/6 = 5/12, but I’m stuck after this.
Vernon
Hi Vernon,
This question involves 2 people working on a job together, so it's perfect for the Work Formula:
Work = (AxB)/(A+B)
A and B represent the amount of time that it takes each person to paint the house:
A = 4 hours
B = 6 hours
Now plug in…
(4x6)/(4+6) = 24/10 hours
2.4 hours = 2 hours 24 minutes
Final Answer: A
Your approach would also get you the correct answer. You’ve calculated the amount of the “job” that gets done in 1 hour. By ‘flipping’ that fraction (re: 5/12 ‘flips’ to 12/5), you end up with the total TIME (in hours) it takes to complete the job.
A Competitive Score vs. the School’s Average Score
Rich,
I am aiming to get into top 20-30 B-schools. Will I need a 700 or higher?
Wilhelmina
Hi Wilhelmina,
While most of the Top-20 US Business Schools have an AVERAGE GMAT score of 700+, that's just an average, NOT a requirement. In real basic terms, the higher your GMAT score, the better your "chances", but small increases are rarely meaningful (a 670, a 680 and a 690 all ‘look’ the same to Admissions Committees); a high GMAT score might also help you qualify for a scholarship. Keep in mind that a strong overall application is what it takes to get into B-School and your GMAT score is just one of the parts of the application. If you have a decent GMAT score, but your application ‘stands out’ in other ways, then you can still get into a Top Business School.
GMAT assassins aren't born, they're made,
Rich
