OG Quant Review #55 & #80

Expert replies

480ocean: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Mon Oct 27, 2008 1:01 pm; Thanked: 1 times

by 480ocean » Wed Feb 04, 2009 8:08 am

#55

If (t-8) is a factor of t^2-kt-48, then k = ?

a. 16
b. 12
c. 2
d. 6
e. 14

#80

If x is to be chosen at random from the set (1,2,3,4) and y is to be chosen at random from the set (5,6,7), what is the probability that xy will be even?

a. 1/6
b. 1/3
c. 1/2
d. 2/3
e. 5/6

Thank you in advance for your help with these questions.

Like this post

Source: Beat The GMAT — Problem Solving | Wed Feb 04, 2009 8:08 am

DeepakR: Master | Next Rank: 500 Posts; Posts: 158; Joined: Tue Sep 30, 2008 8:47 amFollowed by: 1 member; Thanked: 23 times; GMAT Score:660

by DeepakR » Wed Feb 04, 2009 8:38 am

P1#
Since t-8 is a factor put t=8 in the given equation,
t^2-kt-48= 0 hence 64-k8-48=0. So k=2. Ans.) C

P2#
If x is to be chosen at random from the set (1,2,3,4) and y is to be chosen at random from the set (5,6,7), what is the probability that xy will be even?

Since xy = even hence x=even and y=even.

Probablility = P(choosing even number from x) AND Prob(choosing even number from y)= (2/4)*(1/3)=1/6=Ans.) A

- Deepak

Like this post

mdavis: Senior | Next Rank: 100 Posts; Posts: 44; Joined: Thu Nov 20, 2008 2:18 pm; Thanked: 1 times

by mdavis » Wed Feb 04, 2009 9:28 am

An even multiplied by an odd will also equal an even.

I started by finding the probability of choosing x from the set and choosing y from the set. So choosing x is 1/4 and y is 1/3. That equals 1/12 when you multiply them.

I then counted the combinations that would then equal xy being even.

1*6
2*5
2*6
2*7
3*6
4*5
4*6
4*7

So 8 possibilities. Add 1/12 8 times and you get 8/12 or when reduced, 2/3. Answer is D.

Does anyone have a faster way to do this?

Like this post

DeepakR: Master | Next Rank: 500 Posts; Posts: 158; Joined: Tue Sep 30, 2008 8:47 amFollowed by: 1 member; Thanked: 23 times; GMAT Score:660

by DeepakR » Wed Feb 04, 2009 9:40 am

Yes you are correct. I missed that one!!. It would be something like below,

P= (Both x and y even) or (X even and Y odd) or (X odd and Y even)
P=(2/4*1/3) + (2/4*2/3) + (2/4*1/3) = 20/24 = 2/3 D.)

- Deepak

Like this post

gaggleofgirls: Master | Next Rank: 500 Posts; Posts: 138; Joined: Thu Jan 15, 2009 7:52 am; Location: Steamboat Springs, CO; Thanked: 15 times

by gaggleofgirls » Wed Feb 04, 2009 4:08 pm

Another way to do the first problem (#55) is

If you know (t-8) is a factor, then (t-8)(t+x) = t^2-kt-48
48/8 = 6 so (t-8)(t+6) = t^2-kt-48
t^2-2t-48 = t^2-kt-48
k=2

-Carrie

Like this post

480ocean: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Mon Oct 27, 2008 1:01 pm; Thanked: 1 times

by 480ocean » Wed Feb 04, 2009 6:50 pm

DeepakR, Thank you! Your explanation for #55 was a million times easier than the explanation in the book.

Thanks for all the help on #80 too!

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review