What did you learn Today? [GMAT Tips]

Question

What did you learn Today? Keeping track of progress.

Often as we work on a big project, it is easy to lose track of progress and we get caught in the routine when not making progress is acceptable since progress is hard to track. I am thinking we should change it up at least for the GMAT Prep. Instead of mindlessly collapsing into bed or throwing the GMAT book against the wall and going out to dinner, I challenge/encourage you to write down one thing - one sentence or bullet point or paragraph of what you have learned today. What did you not know and discovered? Was there something you struggled with, no matter how big or small? Did you learn a new SC rule or perhaps how to better deal with RC passages? Or did you get caught by a math question trap? Regardless of what it is - post these learning here and after a few weeks, we should have a nice list that I can summarize and combine. This will not only get us a great list of tips, but also will encourage and require us to learn something new each day - make sure you learn something each day about the GMAT. I am happy to start with one of the tips I learned through mistakes:

When checking/plugging numbers, always check for positive, negative, zero, fractions (0.5), and negative fractions (-0.5) to ensure you don't miss something. The harder questions always caught me with -0.5 or something like that - especially true when you have division, which with a number less than 1 but greater than 0 becomes multiplication. I would also read the question very carefully - "If X is an integer" - that's simple but if it said nothing about the integer part, you are guaranteed you will need to check for all these fun numbers

If you have other things you have learned today or earlier, post them here!

pallaviisinha · Answer

While Quant comes with practice, if someone needs to learn all the verbal they need for GMAT, I will recommend egmat. I scored 34 in verbal in 1st GMAT without any preparation. Even though I have been subscribed to egmat since April 2015 (I kept on renewing the subscription), I could not take out time for studies. With 2nd GMAT coming up on 19th August, I feel that e-gmat verbal online will help me achieve the coveted score of 44 in verbal. Keeping my fingers crossed. For Quant, I am heavily dependent on GMAT Club now. However, I used egmat quant online and live for the early phase of preparation.

One thing I would like to share with you to avoid silly mistakes is: after you are done solving the question, go back to the question stem and ensure that the constraint, if any, mentioned in the stem was considered while solving for the question. Inculcate this habit and reap huge benefits. :-)

SouthCity · Answer

There are 5 different ways to find the sum of consecutive integers / integers in Arithmetic Progression
series 
- Some rules apply to only some special cases

An integer series
10,15,20,25,30,35,40,45

1) Sum of the series = 0.5* Total Nos*(First number + last number)

Example: Sum = 0.5*8*(10+45) = 220

2) Sum of the series = Total Nos*Mean = Total Nos * Middle Number

Example : Sum = 8*(25+30)/2=8*27.5 = 220

3) Sum of the series when last number is unknown

Last number = First number + (Total Nos -1)*difference between two numbers

Example : 10+(8-1)*5=45
Next sum can be done as per previous formula

Special Cases

1) Series = All positive odd integers starting from beginning

1, 3, 5, 7,......(Total 20 numbers)

Sum = 20*20 = 400 (Square of Total Numbers=n^2)

2) Series = All positive even integers starting from beginning

2, 4, 6, 8,......(Total 12 Numbers)

Sum = 12*13 = 156 (n=12, n*(n+1))

Cheers and Happy Learning

-South City

AtifS · Answer

So, here is what I just learnt a minute or so ago today.

I was attempting gmat-math-question-of-the-day-111635.html Q of the day in Quants and solved today's Q of the day. I did get it right but wasted time because of being weak at fundamental concepts (It has been years since I looked at them). It was a DS question.

I kept plugging in the numbers e.g. 1, 2, 3, 4, and 5 for both statements, which took about a minute to get to the right answer.
If I hadn't looked at Bunuel's solution, I may not have remembered the simple concept about odd & even numbers. Because, if I had revised the concepts very well then I would have known right away that both statements themselves are sufficient.

Looks like, gotta thoroughly revise all the concepts and math foundations.

"If the base is weak the building is not going to last long."

Kurtosis · Answer

Here is what I learnt today:
1) SC questions that involve comparison are quite tricky and proper care must be taken to ensure that the right things are compared.
2) This was the most challenging question I faced today: isotopes-in-the-recently-excavated-bones-of-a-14-year-old-156846.html
3) I often ignore the 'checks'(eg: negation technique, yes/no check, etc.) when I get stuck between 2 answer choices in CR questions. Applying the 'checks' will help improve my accuracy while solving hard CR questions.

kevinzurek · Answer

I was working on a very difficult Data Sufficiency question today. After about 2 minutes I was able to conclude that (1) was not sufficient and that (2) looked far too time consuming so i guessed E.

The correct answer was B.

ALWAYS guess towards sufficiency, rather than insufficiency. B/A/D are the most common answers to Data Sufficiency questions.

manojISB123 · Answer

Here what I learnt today while solving sequence and series questions. In such question many a times key is to find the pattern. How to find the pattern? Write first few terms(say 4-5) and you will start discovering the pattern. For ex: See this question:
In the sequence 1, 2, 2, …, anan, …, an=an−1∗an−2an=an−1∗an−2. The value of a13 is how many times the value of a11?

a1=1=20a1=1=20 
a2=2=21a2=2=21
a3=a2∗a1=1∗2=21a3=a2∗a1=1∗2=21
a4=a3∗a2=2∗2=22a4=a3∗a2=2∗2=22
a5=a4∗a3=4∗2=23a5=a4∗a3=4∗2=23
a6=a5∗a4=8∗4=25a6=a5∗a4=8∗4=25
a7=a6∗a5=32∗8=28a7=a6∗a5=32∗8=28

If you notice exponents form Fibonacci sequence: {0, 1, 1, 2, 3, 5, 8, ...} (Fibonacci sequence is a sequence where each subsequent number is the sum of the previous two)

So, it will continue as follows: {0, 1, 1, 2, 3, 5, 8, 5+8=13, 8+13=21, 13+21=34, 21+34=55, 34+55=89, 55+89=144, ...}

So PATTERN is KEY ! Pay Attention and make yourself aware of few standard series such as AP, GP, Fibonacci etc..

Thanks
Manoj

bb · Answer

Adding one more for today: 
If you are doing well and heading for 700+ score, you can always expect a questions you won't know how to solve. Try this one for example.

A fast-growing water lily doubles in size every day and it takes 30 days for a water lily to cover the entire surface of the pond. How many days would it take for the water lilly to cover half the pond?

A. 8.33 
B. 12.5 
C. 15 
D. 20 
E. 29

SouthCity · Answer

Standard deviation is one of the favorite topic of test makers for 45+ scorers. There are 6-7 concepts tested in this topic itself. It will help to put them at one place.

1) S.D of a set where the elements are all equal. A = {5.3,5.3,5.3} SD = 0
Note there is no exception to this. It can be integers and non-integers alike. The questions can be twisted from numbered balls with same number written on each one of them to students scoring same marks in an exam.

2) SD of a new set of numbers derived from old set, where a fixed number is added to each number in the old set to derive new set. Please note the wording - a fixed number or same number - No percentage is mentioned here.
A = {6, 2, 8, 7} SD = x, B = {9,5,11,10} SD = ? . You can see same number is added to each element. New SD will be same as old SD.

3) SD of a new set of numbers derived from old set, where a fixed percentage is added to each number in the old set to derive new set. Please note the wording - a fixed percentage is same as "same percentage"mentioned here.
A = {100, 200, 800, 700} SD = x, B = {110,220,880,770} SD = ? . You can see same percentage (=10%) is added to each element. New SD will be same as old SD + 10% increase of SD = 1.1x.

4) Countless question on SD of consecutive integers / numbers in Arithmetic Progression series
a) SD of consecutive integers/ AP series is exactly same regardless of starting point as long as number of elements in a set is same.
For example : A = (10, 15, 20) B = (95,100,105) C = (65,70,75) - All same SD  
b) SD of consecutive integers can be determined if # or No of elements in a series is given. That's all.
c) SD of AP series can be determined if difference between elements and # of elements are given. That's all.

In any of the cases a) b) c), SD is independent of starting integer.
 
5) GMAT Club math book has one excellent section on the new SD of a new set when an element is added to existing set. There are four cases depending on distance of new element from mean a)greater than b) less than c) equal to existing mean or d) distance zero. New SD will be a)greater than b) less than c) equal to existing SD or d) lowest in that order. For details GMAT club mathbook is there for free download.

-South City

AtifS · Answer

Well, that's an interesting as well as a tricky question. At first I was also falling into trap, if I am getting it right this time.

Suppose, if I plant water lily of  1 cm  today then it will be doubled today to  2 cm  and then after tomorrow it will become  4 cm  after doubling again.

Days:  1, 2, 3, 4, 5,....., 30 
Size:  2, 4, 8, 16, 32,.....,30th value  ---> 30th value on 30th day.
Size:  2^1, 2^2, 2^3, 2^4, 2^5, .....,2^ 30

We conclude that in 30 days its size will be  2^30  cm which will cover the whole pond and we can say that size of pond is  2^ 30 cm .
Size of half pond will be -->  \frac{2^ 30}{2}cm = 2^ 29 cm  --->29th value will be on 29th day. It will take  29  days for water lily to cover half of the pond.

Hence, answer should be   E  .

I could easily go for  15  days by falling into the trap but soon I realised that it is doubling every day which means twice of the size of the every previous day.

Let me know if I am right or not. And, how can I make the exponent 30 of any base value together here on GClub as 3 & 0 appear separately in the power?

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