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

Redeem

Gmat club PS.

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Legendary Member
Posts: 1077
Joined: Mon Dec 13, 2010 1:44 am
Thanked: 118 times
Followed by:33 members
GMAT Score:710

Gmat club PS.

by bblast » Mon May 23, 2011 11:39 pm
The probability that a visitor at the mall buys a pack of candy is 30%. If three visitors come to the mall today, what is the probability that exactly two visitors will buy a pack of candy?

0.343
0.147
0.189
0.063
0.027

C

[spoiler]before marking D for this I gave a thought that it should be C

(a,b not c)+(a,c not b) + (b,c not a) = .63+.63+.63. However, I was not sure.
How to decide and mark it in my mind ? I do get mixed up at times despite knowing how to solve any probablilty P&C question.[/spoiler]
Cheers !!

Quant 47-Striving for 50
Verbal 34-Striving for 40

My gmat journey :
https://www.beatthegmat.com/710-bblast-s ... 90735.html
My take on the GMAT RC :
https://www.beatthegmat.com/ways-to-bbla ... 90808.html
How to prepare before your MBA:
https://www.youtube.com/watch?v=upz46D7 ... TWBZF14TKW_
Top
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 905
Joined: Sun Sep 12, 2010 1:38 am
Thanked: 378 times
Followed by:123 members
GMAT Score:760

by Geva@EconomistGMAT » Mon May 23, 2011 11:57 pm
bblast wrote:The probability that a visitor at the mall buys a pack of candy is 30%. If three visitors come to the mall today, what is the probability that exactly two visitors will buy a pack of candy?

0.343
0.147
0.189
0.063
0.027

C

[spoiler]before marking D for this I gave a thought that it should be C

(a,b not c)+(a,c not b) + (b,c not a) = .63+.63+.63. However, I was not sure.
How to decide and mark it in my mind ? I do get mixed up at times despite knowing how to solve any probablilty P&C question.[/spoiler]
Your reasoning is correct: "exactly" type probability questions must account for the order of choosing the events. Exactly two candies could be 1st and 2nd, 1st and 3rd, 2nd and 3rd.

Generally speaking for this (rare) type of questions, the scenarios should be symetrical - each of the three scenarios above will have the same equal probability. Therefore, it is enough to find the probability of a single scenario (say, A, B, not C), and multiply that multiply that fraction by the number of scenarios - in this case, 0.063*3.
Geva
Senior Instructor
Master GMAT
1-888-780-GMAT
https://www.mastergmat.com
Top
User avatar
GMAT Instructor
Posts: 3650
Joined: Wed Jan 21, 2009 4:27 am
Location: India
Thanked: 267 times
Followed by:80 members
GMAT Score:760

by sanju09 » Tue May 24, 2011 12:48 am
bblast wrote:The probability that a visitor at the mall buys a pack of candy is 30%. If three visitors come to the mall today, what is the probability that exactly two visitors will buy a pack of candy?

0.343
0.147
0.189
0.063
0.027

C

[spoiler]before marking D for this I gave a thought that it should be C

(a,b not c)+(a,c not b) + (b,c not a) = .63+.63+.63. However, I was not sure.
How to decide and mark it in my mind ? I do get mixed up at times despite knowing how to solve any probablilty P&C question.[/spoiler]
Probability that exactly two visitors out of three will buy a pack of candy

= 3 (choose) 2 × 0.3 × 0.3 × 0.7

= 3 × 0.063

= [spoiler]0.189


C[/spoiler]
The mind is everything. What you think you become. -Lord Buddha



Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 436
Joined: Tue Feb 08, 2011 3:07 am
Thanked: 72 times
Followed by:6 members

by manpsingh87 » Tue May 24, 2011 1:00 am
https://www.beatthegmat.com/probablity-t81540.html
O Excellence... my search for you is on... you can be far.. but not beyond my reach!
Top
Post new topic Post Reply
• Page 1 of 1