BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Please help with this sum

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Source: — Problem Solving |
User avatar
Legendary Member
Posts: 1309
Joined: Mon Apr 04, 2011 5:34 am
Location: India
Thanked: 310 times
Followed by:123 members
GMAT Score:750

by cans » Sun Jun 05, 2011 4:12 pm
prob that a man can hit = 1/4 (once in 4 shots)
and thus prob that he misses = 1 - 1/4 = 3/4
4 shots fired. let H represents hit and M represents miss
this p(Hit the target) = P(H) + P(MH) + P(MMH) + P(MMMH)
= 1/4 + (3/4)*(1/4) + (3/4)(3/4)(1/4) + (3/4)(3/4)(3/4)(1/4)
1/4 + 3/16 + 9/64 + 27/256 = 175/256

IMO D
P(H) + P(MH) + P(MMH) + P(MMMH) (we get this as if he hits first, he won't need to fire any other shot. MH - misses first and hits in the 2nd. MMH - misses first 2 shots and hits in 3rd. MMMH - misses first 3 shots and hits in the 4th.)
If my post helped you- let me know by pushing the thanks button ;)

Contact me about long distance tutoring!
[email protected]

Cans!!
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Sun Jun 05, 2011 5:31 pm
vinitrathi1 wrote:Q.A man can hit a target once in 4 shots. If he fires 4 shots in succession, what is the probability that he will hit his target?

1. 1
2. 1/256
3. 81/256
4. 175/256
5. 144/256
Another way to solve this is to say that P(hit target at least once in 4 tries) = 1 - P(hit target zero times in 4 tries)

P(hit target zero times in 4 tries)=(3/4)(3/4)(3/4)(3/4)=81/256

So, P(hit target at least once in 4 tries) = 1 - 81/256
=175/256

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Legendary Member
Posts: 1084
Joined: Fri Apr 15, 2011 2:33 pm
Thanked: 158 times
Followed by:21 members

by pemdas » Sun Jun 05, 2011 5:54 pm
Hi Brent, don't we violate independence of events-Hit/Miss here?
P(hit target zero times in 4 tries)=(3/4)(3/4)(3/4)(3/4)=81/256
with each successive try the probability must change, BUT you keep it absolute 3/4 (Miss) at every successive try

i ask to reveal the source
i'm not sure though - OA isn't in line with what i've obtained (in spoiler) it's close to ans. 4 but deviation of 2% (175/256=68%, my calc is 70%)
[spoiler]
i would seek 3/4*2/3*1/2*1 first try
next tries 1/4*2/3*1/2*1, 3/4*1/3*1/2*1 and 3/4*2/3*1/2*1

1/4+1/12+1/8+1/4=(6+2+3+6)/24=17/24[/spoiler]
Success doesn't come overnight!
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 3380
Joined: Mon Mar 03, 2008 1:20 am
Thanked: 2256 times
Followed by:1535 members
GMAT Score:800

by lunarpower » Wed Jun 08, 2011 2:15 am
this is another problem on which the issue is bad wording -- "a man can hit a target once in 4 shots" is ambiguous.
pemdas is interpreting this statement as "out of exactly 4 shots, exactly 1 will hit the target". brent is reading it as "he has a 1/4 chance of hitting the target, regardless of what has already happened."

brent's interpretation is more in line with my own intuition about what the writers *meant* to say, but the question is genuinely ambiguous.

in any case, though --
i can see that some posters (pemdas in particular) don't know the source of the problem.

i'll ask the same question i've asked about a thousand times:
IF YOU DON'T KNOW THE SOURCE, WHY ARE YOU USING THE PROBLEM?

do not use problems from unknown third-party sources -- most of them are TERRIBLE, and many (if not most) of them will basically cause you to un-learn, or get confused about, the things that you're supposed to learn.
Ron has been teaching various standardized tests for 20 years.

--

Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi

--

Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.

Yves Saint-Laurent

--

Learn more about ron
Top
Legendary Member
Posts: 1084
Joined: Fri Apr 15, 2011 2:33 pm
Thanked: 158 times
Followed by:21 members

by pemdas » Wed Jun 08, 2011 10:10 am
Ron, thanks for resolving my doubt.
>> why with so many educated gmatters, we are still having an unknown source issue?
i doubt any one with much less education/schooling than us at BTG, ever buys beans at the groceries store without the origin, expiry codes
Success doesn't come overnight!
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 3380
Joined: Mon Mar 03, 2008 1:20 am
Thanked: 2256 times
Followed by:1535 members
GMAT Score:800

by lunarpower » Thu Jun 09, 2011 1:09 am
pemdas wrote: i doubt any one with much less education/schooling than us at BTG, ever buys beans at the groceries store without the origin, expiry codes
well done.
Ron has been teaching various standardized tests for 20 years.

--

Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions à Ron en français
Voit esittää kysymyksiä Ron:lle myös suomeksi

--

Quand on se sent bien dans un vêtement, tout peut arriver. Un bon vêtement, c'est un passeport pour le bonheur.

Yves Saint-Laurent

--

Learn more about ron
Top
Post new topic Post Reply
• Page 1 of 1