GMATClub m#05

Again got 26/37 correct. This time percentile ranking was 45.2% 60.1% (for some reason, my percentile ranking just changed in that last 1 hour) - haha, I guess I did get a little better.

But I ran out of time on this test - had to pause in order to finish it. Yes, concentration was wavering.

Got frustrated with Probability and P&C questions - I really need to understand this topic before attempting any more questions. Maybe I should ask my school Math tutor if he'd help me out with this topic??? I'm seriously considering it now.

Incorrect Qs Summary:
Topic - No. of questions incorrect
Geo - 1
TSD - 1
Probability/P&C - 2
Inequalities - 1
Number Properties/Is x divisible by y type - 6!

Hmmm...

CP #16 - have not understood why not my way?

Question (from MGMAT Practice Test #1)
In a room filled with 7 people, 4 people have exactly 1 sibling in the room and 3 people have exactly 2 siblings in the room. If two individuals are selected from the room at random, what is the probability that those two individuals are NOT siblings?

Solution
We are told that 4 people have exactly 1 sibling. This would account for 2 sibling relationships (e.g. AB and CD). We are also told that 3 people have exactly 2 siblings. This would account for another 3 sibling relationships (e.g. EF, EG, and FG). Thus, there are 5 total sibling relationships in the group.

Additionally, there are (7 x 6)/2 = 21 different ways to chose two people from the room.

Therefore, the probability that any 2 individuals in the group are siblings is 5/21. The probability that any 2 individuals in the group are NOT siblings = 1 – 5/21 = 16/21.

The correct answer is E. 

CP #13

 From GMATClub's Probability chapter:


Q: There are 5 chairs. Bob and Rachel want to sit such that Bob is always left to Rachel. How many ways it can be done ?
Solution: Because of symmetry, the number of ways that Bob is left to Rachel is exactly 1/2 of all possible ways:
N = 1/2 * 5P2 = 10

Why half?

CP #12

Another one from GMATClub Math Book's Probability chapter.
Given that there are 5 married couples. If we select only 3 people out of 10, what is the probability that none of them are married to each other?

They have given 3 different ways to solve it. All have gone over my head. Help!

CP #11

This one's from the Probability chatper in GMATClub's Math Book.

If the probability of raining on any given day in Atlanta is 40%, what is the probability of raining on exactly 2 days in a 7-day period?

The way I look at it is this:
7 days, out of which on 2 it rains.
2 rain, 2 without rain = 0.4*0.4*0.6*0.6*0.6*0.6*0.6
= (0.4)^2 * (0.6)^5

However, the actual answer is
7C2 * (0.4)^2 * (0.6)^5

I don't understand why the 7C2 is required.

--

Went through the explanation in the chapter itself. It says that, it could rain on Day 1 and 2, or on Day 3 and 7. The sequence changes, although the number of days on which it rained remains the same. This must be factored in to get the correct answer. So other than the probability of it raining on those 2 and 5 days, we must also factor in the possibilites of different days on which it must rain.
At a preliminary level, I get it. But, I'll probably need a few more questions of the type or a different explanation to understand this thoroughly. Thoughts anyone?

1 hr to go to office 20 kms away, 1 hr 15 mins to come back. On an avg, how much time do I waste everyday?

Finished Distance, Speed & Time from a GMATClub post. The tabular representation method is brilliant. It saved me a lot of time, since I don't have to keep writing the different forms of the formula S=D/T for every component of the question. Am definitely using the table form from now on! :-)

Also went through the post on Work problems. Didn't require much effort since my concepts are pretty clear. The only issue I have here is the time I take to solve a question, if I only rely on my concept. I will need to practice a lot of Work problems, and probably revisit my TIME notes for shortcuts. Buit I will get back to this post my diagnostic test, as the diagnostic test is untimed. Plus, there is only a limit to how much I can do in every topic over the next 2 days. Yup, probably giving the test on Saturday. Should revise everything on Friday. Phew...

Update on P&C: went through most of the solved examples in RS Aggarwal, need to wrap up Combinations with the last few questions from the book. Once this is done, need to go back to the GMATClub Math book chapter on Probability and try solving those examples I was having trouble with.

As for Remainders, still need to speak to that friend regarding the questions from the link I did not understand.

The probability of my reading the new Artemis Fowl novel on the weekend is...

Went through Combinatorics and Probability chapters in GMATClub's Math Book. Combi was easy, but probability - there were some methods of solving examples that I couldn't grasp. I've done these topics in school, and I had a brilliant Math tutor who gave me pretty much all the tools required to solve questions of this type.

Am going to go through RS Aggarwal's Class XI book to revise my old way of solving these questions. Hopefully, I'll be able to solve the challenging examples from GMATClub's Math book after that.

Btw, am "working from home" today. Project is more or less done, have to put in around an hour's worth of my time on wrapping it up today. So thought it was best to stick around the house and wrap up Probability during the day...

I actually want to finish up and revise my theory by Friday evening or max Saturday, give the OG Diagnostic Test, and get to reading Atlantis Complex - the new Artemis Fowl novel! I did NOT buy it when I saw it in the book shop on Saturday, because I knew I'd want to read but I need to study now to make up for my loss of 3 weeks. Damn work!