A box contains 100 tickets marked 1 to 100. One ticket is : GMAT Problem Solving (PS)
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# A box contains 100 tickets marked 1 to 100. One ticket is

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A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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07 Feb 2011, 01:33
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A box contains 100 tickets marked 1 to 100. One ticket is picked at random. What is the probability that the number on the ticket will be divisible by either 2 or 3, but not 6?

[Reveal] Spoiler:
51/100

The official solution is the following:

[Reveal] Spoiler:
All possibilities = 100
Numbers divisible by 2 = 50 ... A
Numbers divisible by 3 = 33 ... B
Numbers divisible by 6 = 16 ... C

Positive outcomes = A + B - 2*C

Probability = 51/100

Can someone please explain why they subtract C twice?

Thank you.

Last edited by Bunuel on 07 Jul 2013, 04:43, edited 1 time in total.
Renamed the topic and edited the question.
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Re: Box with 100 tickets marked 1...100 [#permalink]

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07 Feb 2011, 02:24
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nonameee wrote:
The question from a textbook on probability:

A box contains 100 tickets marked 1 to 100. One ticket is picked at random. What is the probability that the number on the ticket will be divisible by either 2 or 3, but not 6?

[Reveal] Spoiler:
51/100

The official solution is the following:

[Reveal] Spoiler:
All possibilities = 100
Numbers divisible by 2 = 50 ... A
Numbers divisible by 3 = 33 ... B
Numbers divisible by 6 = 16 ... C

Positive outcomes = A + B - 2*C

Probability = 51/100

Can someone please explain why they subtract C twice?

Thank you.

A box contains 100 tickets marked 1 to 100. One ticket is picked at random. What is the probability that the number on the ticket will be divisible by either 2 or 3, but not 6?

Total = 100 numbers.

Numbers divisible by 2 = 50 ... A
Numbers divisible by 3 = 33 ... B
Numbers divisible by 6 = 16 ... C

Now, both A (numbers divisible by 2) and B (numbers divisible by 3) contain C (numbers divisible by 6) so to get the numbers which are divisible by either 2 or 3, but not 6 we should subtract C twice, once for A and once for B: A + B - 2*C= 51

Probability = 51/100

If it were: "What is the probability that the number on the ticket will be divisible by either 2 or 3 (or both)?" Then we would subtract C only once to get rid of double counting.
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Re: Box with 100 tickets marked 1...100 [#permalink]

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07 Feb 2011, 03:12
Bunuel, thanks. I got it.
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Re: Box with 100 tickets marked 1...100 [#permalink]

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07 Feb 2011, 05:09
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I thought it to be 67/100 and realized my mistake.

Count of numbers divisible by 2 = 50. In these 50 numbers there are 16 numbers that are divisible by 6 and we must take them out. 50-16=34

Count of numbers divisible by 3 = 33. In these 33 numbers there are 16 numbers that are divisible by 6 and we must take them out. 33-16=17

Total numbers divisible by 2 or 3 but not 6 = 34+17=51

Probability = 51/100.

Good one.
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Re: Box with 100 tickets marked 1...100 [#permalink]

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08 Feb 2011, 11:11
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Re: A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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25 Nov 2013, 07:26
nonameee wrote:
A box contains 100 tickets marked 1 to 100. One ticket is picked at random. What is the probability that the number on the ticket will be divisible by either 2 or 3, but not 6?

[Reveal] Spoiler:
51/100

The official solution is the following:

[Reveal] Spoiler:
All possibilities = 100
Numbers divisible by 2 = 50 ... A
Numbers divisible by 3 = 33 ... B
Numbers divisible by 6 = 16 ... C

Positive outcomes = A + B - 2*C

Probability = 51/100

Can someone please explain why they subtract C twice?

Thank you.

Every 6 numbers there are 3 divisible by 2 or 3 and not divisible by 6
Then in 100 numbers there are 16 sets of 6 and 4 numbers left as remainder.
So, 16(3) = 48. Now of the numbers remaining (92,98,99 and 100) there 3 of them are multiples of 2 or 3 but not 6.
So total is 51 numbers
Hence prob is 51/100

Hope it helps
Cheers
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Re: A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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01 Jun 2014, 07:59
Is it the same as saying: divisible by either 2 or 3 but not by both?
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Re: A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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01 Jun 2014, 08:49
Enael wrote:
Is it the same as saying: divisible by either 2 or 3 but not by both?

_______
Yes, it is the same.
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Re: A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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14 Jul 2015, 05:55
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Re: A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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22 May 2016, 02:32
nonameee wrote:
A box contains 100 tickets marked 1 to 100. One ticket is picked at random. What is the probability that the number on the ticket will be divisible by either 2 or 3, but not 6?

[Reveal] Spoiler:
51/100

The official solution is the following:

[Reveal] Spoiler:
All possibilities = 100
Numbers divisible by 2 = 50 ... A
Numbers divisible by 3 = 33 ... B
Numbers divisible by 6 = 16 ... C

Positive outcomes = A + B - 2*C

Probability = 51/100

Can someone please explain why they subtract C twice?

Thank you.

51/100 wud be the soln
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Re: A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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22 May 2016, 09:59
1
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nonameee wrote:
A box contains 100 tickets marked 1 to 100. One ticket is picked at random. What is the probability that the number on the ticket will be divisible by either 2 or 3, but not 6?

[Reveal] Spoiler:
51/100

The official solution is the following:

[Reveal] Spoiler:
All possibilities = 100
Numbers divisible by 2 = 50 ... A
Numbers divisible by 3 = 33 ... B
Numbers divisible by 6 = 16 ... C

Positive outcomes = A + B - 2*C

Probability = 51/100

Can someone please explain why they subtract C twice?

Thank you.

Total numbers= 100
numbers divisible by 2= 50
numbers divisible by three= 33
numbers divisible by 6= 16

numbers divisible by 2 and 3 but not 6= 50-16+33-16= 51

probability= 51/100
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Re: A box contains 100 tickets marked 1 to 100. One ticket is [#permalink]

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27 Aug 2016, 08:38
Please update the options for this question.
Correct Ans should be 51/100.
No. of multiples of 2 = 50
No. of multiples of 3 = 33
No. of multiples of 6 = 16
Since both multiples of 2 & 3 will also contain multiples of 6 so we need to subtract it twice.
Total no. of multiples of 2 & 3 but not 6 = 51.
Probability = 51/100
Re: A box contains 100 tickets marked 1 to 100. One ticket is   [#permalink] 27 Aug 2016, 08:38
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