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Manager  Joined: 11 Feb 2008
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Every trading day, the price of CF Corp stock either goes up  [#permalink]

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Difficulty:   35% (medium)

Question Stats: 74% (01:52) correct 26% (02:04) wrong based on 220 sessions

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(A) 1/16
(B) 1/8
(C) 5/32
(D) 9/32
(E) 3/8

The price is up by exactly $3 in 5 days means that 4 days it went up (+4$) and one day it went down (-1$). UUUUD --> $$P(U=4)=C^4_5*(\frac{1}{2})^4*\frac{1}{2}=\frac{5}{32}$$. Answer: C. If the probability of a certain event is $$p$$, then the probability of it occurring $$k$$ times in $$n$$-time sequence is: $$P = C^k_n*p^k*(1-p)^{n-k}$$ In our case probability of having 4 days when price went up (4U) and one day when price went down (D): $$n=5$$ (5 days); $$k=4$$ (4 days when price wen t up); $$p=\frac{1}{2}$$ (probability of price going up is 1/2). So, $$P = C^k_n*p^k*(1-p)^{n-k}=C^4_5*(\frac{1}{2})^4*(1-\frac{1}{2})^{(5-4)}=C^4_5*(\frac{1}{2})^5$$ OR: probability of scenario U-U-U-U-D is $$(\frac{1}{2})^4*(\frac{1}{2})^4=(\frac{1}{2})^5$$, but U-U-U-U-D can occur in different ways: U-U-U-U-D; U-U-U-D-U U-U-D-U-U U-D-U-D-U D-U-U-U-U; 5 ways (# of permutations of five letters U-U-U-U-D, which is $$\frac{5!}{4!}=5=C^4_5$$). Hence $$P=\frac{5!}{4!}*(\frac{1}{2})^5$$. Check this links for similar problems: viewtopic.php?f=140&t=96468&p=742767&hilit=+google#p742767 viewtopic.php?f=140&t=56812&hilit=+probability+occurring+times viewtopic.php?f=140&t=88069&hilit=+probability+occurring+times viewtopic.php?f=140&t=87673&hilit=+probability+occurring+times Also you can check Probability chapter of Math Book for more (link in my signature). Hope it helps. _________________ ##### General Discussion Senior Manager  Status: Math is psycho-logical Joined: 07 Apr 2014 Posts: 402 Location: Netherlands GMAT Date: 02-11-2015 WE: Psychology and Counseling (Other) Re: Every trading day, the price of CF Corp stock either goes up [#permalink] ### Show Tags Hmmm, I have a very basic problem with this question... Why is it that if we want the stock to go up exactly 3 points we need it to go up for 4 out of 5 days and not 3? Does it start by zero, so first day is zero, second 1, third 2 and fourth day is 3? Would it be possible to also solve it using probability? Thank you. Math Expert V Joined: 02 Sep 2009 Posts: 59588 Re: Every trading day, the price of CF Corp stock either goes up [#permalink] ### Show Tags pacifist85 wrote: Hmmm, I have a very basic problem with this question... Why is it that if we want the stock to go up exactly 3 points we need it to go up for 4 out of 5 days and not 3? Does it start by zero, so first day is zero, second 1, third 2 and fourth day is 3? Would it be possible to also solve it using probability? Thank you. It does not matter what is the initial price. We know that every trading day, the price, whatever it is, either goes up by$1 or goes down by $1 and the question asks about the probability that the price is up by exactly$3 from its initial price at the end of 5 trading days.
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Every trading day, the price of CF Corp stock either goes up  [#permalink]

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Hi pacifist85,

The specific details in this question factor in a great deal in how you must go about solving it.

We're told that the price of the stock either increases by $1 OR decreases by$1 each day for 5 days. So EVERY DAY the price WILL change.

To have a net increase of $3, 3 days of increases will NOT be enough. Here's why: 5 days total 3 days of increases = +$3

BUT the remaining 2 days will then be decreases...
2 days of decreases = -$2 Net effect: +$3 - $2 = +$1
This is NOT what we're looking for, so 3 days of increases would NOT be enough.

With 4 days of increases though...
4 days of increase = +$4 1 day of decrease = -$1

Net effect: +$4 -$1 = +$3 And then there 5 days of increases, which = +$5

So, to get a net increase of $3, we need 4 days of increases (out of the 5 total days). GMAT assassins aren't born, they're made, Rich _________________ Originally posted by EMPOWERgmatRichC on 02 Feb 2015, 13:22. Last edited by EMPOWERgmatRichC on 19 Feb 2016, 13:25, edited 1 time in total. Senior Manager  Joined: 15 Oct 2015 Posts: 298 Concentration: Finance, Strategy GPA: 3.93 WE: Account Management (Education) Re: Every trading day, the price of CF Corp stock either goes up [#permalink] ### Show Tags anaik100 wrote: Every trading day, the price of CF Corp stock either goes up by$1 or goes down by $1 with equal likelihood. At the end of 5 trading days, what is the probability that the price of CF Corp stock is up by exactly$3 from its initial price?

(A) 1/16
(B) 1/8
(C) 5/32
(D) 9/32
(E) 3/8

any brute force method here plzzzzzz?
EMPOWERgmat Instructor V
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: Every trading day, the price of CF Corp stock either goes up  [#permalink]

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2
HI Nez,

Since each day involves either an increase or a decrease, there are 2^5 = 32 possible arrangements. Brute-forcing EVERY one of those 32 options isn't really practical though. If you do a bit of work first to figure out what it would take for a 3 dollar increase to occur (4 increases and 1 decrease), THEN you can brute force the possible options from there.

GMAT assassins aren't born, they're made,
Rich
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Re: Every trading day, the price of CF Corp stock either goes up  [#permalink]

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anaik100 wrote:
Every trading day, the price of CF Corp stock either goes up by $1 or goes down by$1 with equal likelihood. At the end of 5 trading days, what is the probability that the price of CF Corp stock is up by exactly $3 from its initial price? (A) 1/16 (B) 1/8 (C) 5/32 (D) 9/32 (E) 3/8 In order to have a net change of +$3, we must have 4 “up” days and 1 “down” day. Thus one of the sequences of 4 ups and 1 down is:
up - up - up - up - down or UUUUD

Let’s determine the probability of this sequence:

P(UUUUD) = (1/2)^5 = 1/32

However, since a sequence of the 4 ups and 1 down (UUUUD) can be arranged in 5C4 = 5!/4! = 5 ways, the overall probability is 5 x 1/32 = 5/32.

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Re: Every trading day, the price of CF Corp stock either goes up  [#permalink]

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_________________ Re: Every trading day, the price of CF Corp stock either goes up   [#permalink] 14 Apr 2019, 23:17
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