Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 24 May 2017, 14:42

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# What is the probability that a field gun will hit thrice on

Author Message
TAGS:

### Hide Tags

Manager
Joined: 29 Nov 2011
Posts: 80
Followers: 1

Kudos [?]: 322 [0], given: 37

What is the probability that a field gun will hit thrice on [#permalink]

### Show Tags

09 Feb 2012, 23:35
5
This post was
BOOKMARKED
00:00

Difficulty:

95% (hard)

Question Stats:

33% (01:52) correct 67% (01:15) wrong based on 179 sessions

### HideShow timer Statistics

What is the probability that a field gun will hit thrice on target in five tryouts?

(1) The field gun maintains a history of hitting once on target in every five tryouts.
(2) The probability that the field gun will hit twice on target in five tryouts is 0.2048.
[Reveal] Spoiler: OA
Math Expert
Joined: 02 Sep 2009
Posts: 38858
Followers: 7727

Kudos [?]: 106044 [0], given: 11607

Re: What is the probability that a field gun will hit thrice on [#permalink]

### Show Tags

10 Feb 2012, 12:53
Expert's post
3
This post was
BOOKMARKED
I think the wording of this question is not the one the realistic GMAT question would use, so I don't like this problem at all. Anyway:

What is the probability that a field gun will hit thrice on target in five tryouts?

If the probability of a certain event is $$p$$, then the probability of it occurring $$k$$ times in $$n$$-time sequence is: $$P = C^k_n*p^k*(1-p)^{n-k}$$ (binomial distribution).

For our case we are asked to calculate $$P = C^3_5*p^3*(1-p)^{2}$$. So all we need is the value of p.

(1) The field gun maintains a history of hitting once on target in every five tryouts --> p=1/5. Sufficient.

(2) The probability that the field gun will hit twice on target in five tryouts is 0.2048 --> $$P = C^2_5*p^2*(1-p)^{3}$$ --> we'll get 3 values for p: one negative (not valid) and two positive (0.2 and ~0.63). Not sufficient.

For more on this subject check Probability chapter of Math Book: math-probability-87244.html

Questions about the same concept to practice:
there-is-a-90-chance-that-a-registered-voter-in-burghtown-56812.html#p717422
in-how-many-different-ways-can-the-letters-a-a-b-91460.html

Hope it helps.
_________________
Senior Manager
Joined: 19 Oct 2010
Posts: 262
Location: India
GMAT 1: 560 Q36 V31
GPA: 3
Followers: 7

Kudos [?]: 80 [0], given: 27

Re: What is the probability that a field gun will hit thrice on [#permalink]

### Show Tags

03 Jul 2012, 09:58
Bunuel, can you suggest an alternate method to approach this problem?
_________________

petrifiedbutstanding

Director
Joined: 22 Mar 2011
Posts: 612
WE: Science (Education)
Followers: 101

Kudos [?]: 947 [0], given: 43

Re: What is the probability that a field gun will hit thrice on [#permalink]

### Show Tags

07 Jul 2012, 07:19
Smita04 wrote:
What is the probability that a field gun will hit thrice on target in five tryouts?

(1) The field gun maintains a history of hitting once on target in every five tryouts.
(2) The probability that the field gun will hit twice on target in five tryouts is 0.2048.

This cannot be a real GMAT test question. The probability of hitting twice the target in 5 tryouts is not complementary to the probability of hitting the target three times in 5 tryouts. From the equation presented by Bunuel, it follows that between 0 and 1, there are two solutions for p. Moreover, the method of proving/finding the roots of that equation, is beyond the scope of the GMAT.

I guess the author of the question meant a D answer, as it can be easily checked that 1/5 is a solution of the equation in (2), and the same as the probability in (1). But probably he/she didn't think further, that there might be another valid solution.
_________________

PhD in Applied Mathematics
Love GMAT Quant questions and running.

CEO
Joined: 17 Nov 2007
Posts: 3586
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40
Followers: 573

Kudos [?]: 3975 [0], given: 360

Re: What is the probability that a field gun will hit thrice on [#permalink]

### Show Tags

07 Jul 2012, 08:00
It's definitely not a GMAT question. Even the first statement "The field gun maintains a history of hitting once on target in every five tryouts." doesn't define the probability of hitting the target with the gun. History can be just 5 shots or 5000 shots but you need infinite historical data to say that p=1/5.
_________________

HOT! GMAT TOOLKIT 2 (iOS) / GMAT TOOLKIT (Android) - The OFFICIAL GMAT CLUB PREP APP, a must-have app especially if you aim at 700+ | PrepGame

Manager
Joined: 13 May 2010
Posts: 122
Followers: 0

Kudos [?]: 17 [0], given: 4

Re: What is the probability that a field gun will hit thrice on [#permalink]

### Show Tags

29 Oct 2012, 13:41
Probability of getting exactly k results out of n trails = nCk/2^n (isn't this problem the same as getting three heads in five coin flips)

Can't we use the above formula to calculate the prob. for k= 3 hits

5C3/2^5

Math Expert
Joined: 02 Sep 2009
Posts: 38858
Followers: 7727

Kudos [?]: 106044 [0], given: 11607

Re: What is the probability that a field gun will hit thrice on [#permalink]

### Show Tags

29 Oct 2012, 13:49
teal wrote:
Probability of getting exactly k results out of n trails = nCk/2^n (isn't this problem the same as getting three heads in five coin flips)

Can't we use the above formula to calculate the prob. for k= 3 hits

5C3/2^5

The probability of success is 1/5 not 1/2, thus your solution is not correct.

Check here: what-is-the-probability-that-a-field-gun-will-hit-thrice-on-127334.html#p1042999 (also check the links provided there for better understanding of the concept).
_________________
Re: What is the probability that a field gun will hit thrice on   [#permalink] 29 Oct 2012, 13:49
Similar topics Replies Last post
Similar
Topics:
2 A rectangular field is 3.2 yards long. What is the area of the field i 1 29 Apr 2017, 00:13
1 What is the area of a rectangular field? 3 21 Mar 2017, 10:10
1 What is the probability that it will rain tomorrow ? 7 04 Aug 2016, 08:00
37 What is the probability that rs=r? 16 02 Feb 2017, 11:53
7 What is the probability that there is exactly 1 defective 3 13 Sep 2016, 08:34
Display posts from previous: Sort by