11 Comments:

 PowerYogi said...

dave, how do you arrive at 9 as the answer if you do the multiplication by 4 thing:

a-4b=15 - (1)
4a-b=15 - (2)

(1)* 4 ==> 4a-16b=60 - (3)
(3)-(2)==> -16b+b = 60-15 ==> b=-3 - (4)
from (4) & (1) ==> a -(-3) = 15 ==> a = 3

a-b = 3 -(-3) => 6 !

Similarly,
from (1) : a = 15 + 4b - (5)
(5) & (2) : 60 + 16b -b = 15 => 15b = -45 => b = -3.
and as above, a = 3.

I think in this case there was just a simple calculation error on your part. Solved anyways the problem SHOULD and WILL give you the same answer. So, don't worry about figuring out which way a problem should be solved. all roads will lead to the same place, just go with the technique you are comfortable with.

4:48 PM  

 PowerYogi said...

oops, one mistake. read above as :

from (4) & (1) => a-4(-3)=15 ==> a = 3;

4:56 PM  

 Wakechick said...

Stay positive, Dave! I know where I screw up math problems is when I overthink them or try to get too fancy. Here's how I would solve your example. You have two variables and two equations, so I'd use algebra and simultanteously solve (I think that's the term) for both variables and then find out what a-b is.

Change the first equation to a=4b+15. Plug that into the 2nd equation.

4(4b+15)-b=15 Now I solve for b and get b=-3.
16b+60-b=15 15b=-45 b=-3
Plug that into the first equation.

a=4(-3)+15 Solve for a and get a=3.

Plugging in to a-b=? 3-(-3)= 6

This is similar to the first method you described, but a different result so you may have just made a minor math error. Just try to stay patient and check your work. I know it's difficult when you have to worry about pacing. Easier said than done, right :) Hang in there buddy!

5:01 PM  

 Wakechick said...

PowerYogi and I, posting simultaneously. Great minds think alike, LOL

5:02 PM  

 aregon23 said...

Sorry but the best way to solve this is *NOT* to find the values of A and B and then subtract one from the other.

What you need to look for is patterns.

For example
a - 4b = 15
4b - b = 15
------------
5a - 5b = 30
5(a - b)= 5(6)

Therfore a-b = 6

No complex calculations where you make mistakes. Interestingly this is exactly similar to the problem I got in the actual GMAT, it went something like this:
a + 2b + 3c = 12 & 3a + 2b + c = 0, what is the value of a + b + c?

In which case, the method shown above will be the best. :)

5:29 PM  

 aregon23 said...

Dave,

I know this is a little late in the day and you might not want to be burdened by any extra information, but I think this is an important tool for verbal. It is called the "Pascal's triangle" Not sure if you are familiar with it, but in case you aren't it is a very simple triangle of numbers that you add up in less than a couple of seconds. It helps you calculate answers for all combination problems and all the probability problems where there are two outcomes and the probability of each is 1/2. Example coins.

Let me know if you want me send you an email with some links on this. Shouldn't take you more than a couple of mins to learn how to operate this triangle. After which you are the king of all the coin toss problems. :)

5:34 PM  

 Dave for MBA said...

Thanks everyone for the feedback and encouragement. Reading your guys support, lifted my spirits. And Aregon... yeah I'd like to see that triangle concept. Go ahead and e-mail me the links and I'll post them on my GMAT Resources side bar.

Thanks.

7:07 PM  

 aregon23 said...

Here ya go! (Links explaining Pascal's triangle, only read the P & C part, ignore the binomial/fibonacchi etc.)
http://www.krysstal.com/binomial.html
http://mathforum.org/dr.math/faq/faq.pascal.triangle.html

It is bloody easy to use, and MUST be used in case of any Probability problem with less than 7 throws of the coin, or any Combination problem with a small number < 8 items.

Let me know if you need any help with Math.

Here is another tip: To recognize multiples of -
2: Last digit is even
3: Sum of digits is a multiple of 3
4: Last 2 digits are a multiple of 4
5: Last digit is a 5 or a 0
6: Sum of digits is a multiple of 3 and last digit is even
9: Sum of digits is a multiple of 9
10: Last digit is 0
12: Sum of digits is a multiple of 3 and the last 2 digits are a multiple of 4.

P.S' Pascal's triangle is used in Quant not verbal as the typo in my first comment claimed, but I am sure you already figured that out. :)

10:33 PM  

 Anonymous said...

All the best. I have my GMAT in the first week of August. So both of us are in the same boat.

8:19 AM  

 Anonymous said...

hi dave

this is aejaz from gmatclub. I think a 700 score is more about having a strict discipline of practice, error review and understanding core concepts, not just particular problems.

please keep taking our challenges and study hard on those mistakes. review them every single day. if you need any help, you are most welcome to ask any questions on gmatclub.

hang in there friend
aejaz

12:16 AM  

 Anonymous said...

Well Dave, different people have different weaknesses. Maybe you feel you're weak in Quant but for me mate...I kinda screw up big time in English. Sentence correction, Critical Reasoning and reading comprehension...simple mistakes and they all are repeated again and again. :-((

8:26 AM  

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