gprep 3 ps

Expert replies

abhasjha: Master | Next Rank: 500 Posts; Posts: 379; Joined: Tue Sep 30, 2008 7:17 amFollowed by: 11 members; Location: NY; Thanked: 28 times

by abhasjha » Wed Jul 23, 2014 5:39 am

If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and not on the last 2 flips?

A. 3/5
B. 1/2
C. 1/5
D. 1/8
E. 1/32

why cant i use = nCr * p^r * q^(n-r)???

https://graceeducation1613.wordpress.co ... questions/

Like this post

Source: Beat The GMAT — Problem Solving | Wed Jul 23, 2014 5:39 am

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Wed Jul 23, 2014 6:19 am

abhasjha wrote:If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and not on the last 2 flips?

A. 3/5
B. 1/2
C. 1/5
D. 1/8
E. 1/32

P(H on the 1st flip) = 1/2.
P(H on the 2nd flip) = 1/2.
P(H on the 3rd flip) = 1/2.
P(T on the 4th flip) = 1/2.
P(T on the 5th flip) = 1/2.
Since we want all of these events to happen, we multiply the fractions:
1/2 * 1/2 * 1/2 * 1/2 * 1/2 = 1/32.

The correct answer is E.

Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3

Like this post

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Wed Jul 23, 2014 6:32 am

abhasjha wrote: why cant i use = nCr * p^r * q^(n-r)???

This formula would be appropriate if the question stem asked the following:
If the coin is flipped 5 times, what is the probability that it will land heads up EXACTLY 3 TIMES?
Here, good outcomes would include the following:
HHHTT
HTHHT
HTTHH.

The GMAT Prep problem is more constrained.
It asks for the probability that the only the FIRST 3 FLIPS are heads, implying that there is only ONE good outcome:
HHHTT.
Thus, the formula above is not appropriate.

Like this post

GMATinsight: Legendary Member; Posts: 1100; Joined: Sat May 10, 2014 11:34 pmFollowed by: 24 members; Location: New Delhi, India; Thanked: 205 times

by GMATinsight » Wed Jul 23, 2014 9:31 am

abhasjha wrote:If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and not on the last 2 flips?

A. 3/5
B. 1/2
C. 1/5
D. 1/8
E. 1/32

why cant i use = nCr * p^r * q^(n-r)???

Given: Probability of Head in 1 flip = 1/2 i.e. Coin is Fair

Probability = Favorable Outcome/Total Outcomes

The favorable Outcomes = 1 [i.e. HHHTT]
Total Outcomes = 2^5 = 32

Probability = 1/32

Answer: Option E

"GMATinsight"Bhoopendra Singh & Sushma Jha
Most Comprehensive and Affordable Video Course 2000+ CONCEPT Videos and Video Solutions
Whatsapp/Mobile: +91-9999687183 l [email protected]
Contact for One-on-One FREE ONLINE DEMO Class Call/e-mail
Most Efficient and affordable One-On-One Private tutoring fee - US$40-50 per hour

Like this post

GMATinsight: Legendary Member; Posts: 1100; Joined: Sat May 10, 2014 11:34 pmFollowed by: 24 members; Location: New Delhi, India; Thanked: 205 times

by GMATinsight » Wed Jul 23, 2014 9:35 am

abhasjha wrote: why cant i use = nCr * p^r * q^(n-r)???

You can't use 5C3 here because 5C3 randomly selects 3 placements of Heads out of 5 placements in 5 flips and doesn't ensure that the Heads are obtained on first three outcomes and on last two

Like this post

abhasjha: Master | Next Rank: 500 Posts; Posts: 379; Joined: Tue Sep 30, 2008 7:17 amFollowed by: 11 members; Location: NY; Thanked: 28 times

by abhasjha » Wed Jul 23, 2014 10:22 am

Thank you mitch and bhoopendra for clearing my doubt as to why 5c3... will not work .

https://graceeducation1613.wordpress.co ... questions/

Like this post

GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Wed Jul 23, 2014 12:11 pm

abhasjha wrote:If a certain coin is flipped, the probability that the coin will land heads is 1/2. If the coin is flipped 5 times, what is the probability that it will land heads up on the first 3 flips and not on the last 2 flips?

A. 3/5
B. 1/2
C. 1/5
D. 1/8
E. 1/32

P(HEADS on 1st AND HEADS on 2nd AND HEADS on 3rd AND TAILS on 4th AND TAILS on 5th) = P(HEADS on 1st) x P(HEADS on 2nd) x P(HEADS on 3rd) x P(TAILS on 4th) x P(TAILS on 5th)
= 1/2 x 1/2 x 1/2 x 1/2 x 1/2
= [spoiler]1/32 = E[/spoiler]

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

Like this post

GMAT/MBA Expert

[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Wed Jul 23, 2014 11:12 pm

Hi abhasjha,

On probability, permutation and combination questions, it's essential to note the details so that you can set up whatever "math" is necessary to get to the correct answer. GMAT questions are carefully written to give you exactly what you need to know to do the work, so make sure that you're taking the proper notes, drawing pictures, etc. to keep track of everything.

In this prompt, I would write down:

HHHTT

So that I can remember what my "goal" is when solving.

You're clearly smart enough to answer many of these questions. To prove to Top Business Schools that you belong, you also have to show that you're organized.

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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