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# M22-08

Author Message
Manager
Joined: 06 Jun 2010
Posts: 158

Kudos [?]: 20 [0], given: 151

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01 Mar 2013, 06:58
What is the probability that one of the two integers randomly selected from range 20-29 is prime and the other is a multiple of 3?

(The numbers are selected independently of each other, i.e. they can be equal)

0.06
0.12
0.15
0.18
0.20

I got the answer as 0.06 but im not sure why its wrong.

Kudos [?]: 20 [0], given: 151

Director
Status: Tutor - BrushMyQuant
Joined: 05 Apr 2011
Posts: 622

Kudos [?]: 770 [0], given: 59

Location: India
Concentration: Finance, Marketing
Schools: XLRI (A)
GMAT 1: 700 Q51 V31
GPA: 3
WE: Information Technology (Computer Software)

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01 Mar 2013, 08:37
shreerajp99 wrote:
What is the probability that one of the two integers randomly selected from range 20-29 is prime and the other is a multiple of 3?

(The numbers are selected independently of each other, i.e. they can be equal)

0.06
0.12
0.15
0.18
0.20

I got the answer as 0.06 but im not sure why its wrong.

Probability that one integer is a prime number
prime numbers between 20-29 are 23,29
So probability is 2/10 = 0.2

Multiple of 3
Multiple's of 3 between 20 and 29 are 21,24,27
so 3/10

Probability of choosing one prime and one factor of 3= (probability of choosing prime first and factor of 3 second) + (Probability of choosing factor of 3 first and prime second)
= 2 * 0.2 *0.3 = 0.12

Hope if helps!
_________________

Ankit

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How to Solve :
Statistics || Reflection of a line || Remainder Problems || Inequalities

Kudos [?]: 770 [0], given: 59

Manager
Joined: 06 Jun 2010
Posts: 158

Kudos [?]: 20 [0], given: 151

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01 Mar 2013, 08:53
Hi,

My question is why are we doing it twice?

Kudos [?]: 20 [0], given: 151

Director
Status: Tutor - BrushMyQuant
Joined: 05 Apr 2011
Posts: 622

Kudos [?]: 770 [1], given: 59

Location: India
Concentration: Finance, Marketing
Schools: XLRI (A)
GMAT 1: 700 Q51 V31
GPA: 3
WE: Information Technology (Computer Software)

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01 Mar 2013, 09:00
1
KUDOS
shreerajp99 wrote:
Hi,

My question is why are we doing it twice?

Because the sequence in which you choose does matter!
_________________

Ankit

Check my Tutoring Site -> Brush My Quant

GMAT Quant Tutor
How to start GMAT preparations?
How to Improve Quant Score?
Gmatclub Topic Tags
Check out my GMAT debrief

How to Solve :
Statistics || Reflection of a line || Remainder Problems || Inequalities

Kudos [?]: 770 [1], given: 59

Intern
Joined: 20 Oct 2011
Posts: 12

Kudos [?]: 1 [0], given: 1

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21 Mar 2013, 17:39
Another way to think of it is:
How many sets of 2 numbers can be formed such that one of them is prime and the other is a multiple of 3? it's twice of (no. of primes x no. of multiples of 3)=12
How many sets of 2 numbers can be formed from the pool of numbers you're allowed to choose from? it's 10x10=100
so probability =12/100=0.12

Kudos [?]: 1 [0], given: 1

Re: M22-08   [#permalink] 21 Mar 2013, 17:39
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# M22-08

Moderator: Bunuel

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