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# The first flight out of Phoenix airport had a late departure

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The first flight out of Phoenix airport had a late departure [#permalink]  12 Jan 2012, 06:51
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Question Stats:

56% (01:58) correct 44% (00:55) wrong based on 43 sessions
The first flight out of Phoenix airport had a late departure. If the next three flights departed on-time, how many subsequent flights need to depart from Phoenix on-time, for the airport's on-time departure rate to be higher than 90%?

A. 6
B. 7
C. 9
D. 10
E. 11

I will see what is the quickest way to solve it then I will provide the explanation
[Reveal] Spoiler: OA

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Re: first flight out of phoenix [#permalink]  13 Jan 2012, 03:28
1/(3+x) <= 0.1
=> x => 7

Therefore (B)
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Re: first flight out of phoenix [#permalink]  06 Mar 2012, 21:25
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My approach:

If the first (one) flight had a late departure, then 1 flight out of 1 flight had a late departure (100% of the flights so far). Then, the stem says that the next 3 flights departed on-time, meaning that 1 out of 4 flights had a late departure (25% of the flights so far). Therefore, following this logic, if we add another 6 on-time planes, we will know that 1 out of 10 flights had a late departure (thus 10% was late and 90% was on-time).

The question asks how many subsequent flights need to depart from Phoenix on-time, for the airport's on-time departure rate to be HIGHER than 90%. As we have seen so far, if 6 subsequent planes were on-time, we would have a rate of EXACTLY 90% of late departure. Therefore, for the airport's on-time departure rate to be HIGHER than 90%, we need 7 subsequent planes to be on-time (1 out of 11 had a late departure or aprox. 9% of the flights -- the remaining 91% is the rate of on-time flights).

Ans: B

Last edited by pedroca on 07 Mar 2012, 05:38, edited 1 time in total.
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Re: The first flight out of Phoenix airport had a late departure [#permalink]  06 Mar 2012, 21:55
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Expert's post
manalq8 wrote:
The first flight out of Phoenix airport had a late departure. If the next three flights departed on-time, how many subsequent flights need to depart from Phoenix on-time, for the airport's on-time departure rate to be higher than 90%?

A. 6
B. 7
C. 9
D. 10
E. 11

I will see what is the quickest way to solve it then I will provide the explanation

The following approach might be the easiest one and less error prone.

We need on-time departure rate to be higher than 9/10, so it should be at least 10/11, which means that 10 out of 11 flights must depart on time. Since for now 3 out of 4 flights departed on time then 10-3=7 subsequent flights need to depart on-time.

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Re: The first flight out of Phoenix airport had a late departure [#permalink]  11 Feb 2013, 09:18
Bunuel wrote:
manalq8 wrote:
The first flight out of Phoenix airport had a late departure. If the next three flights departed on-time, how many subsequent flights need to depart from Phoenix on-time, for the airport's on-time departure rate to be higher than 90%?

A. 6
B. 7
C. 9
D. 10
E. 11

I will see what is the quickest way to solve it then I will provide the explanation

The following approach might be the easiest one and less error prone.

We need on-time departure rate to be higher than 9/10, so it should be at least 10/11, which means that 10 out of 11 flights must depart on time. Since for now 3 out of 4 flights departed on time then 10-3=7 subsequent flights need to depart on-time.

one flight was already late & 3 flight were on time departure so now to have a on-time departure rate to be higher than 9/10,

3 on time + 6 yet to be on time = 9 on time. total of 10, 9 on time but it not higher than 9/10 so so we went for 7 by considering 10/11?
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Re: The first flight out of Phoenix airport had a late departure [#permalink]  11 Feb 2013, 09:31
Expert's post
FTG wrote:
Bunuel wrote:
manalq8 wrote:
The first flight out of Phoenix airport had a late departure. If the next three flights departed on-time, how many subsequent flights need to depart from Phoenix on-time, for the airport's on-time departure rate to be higher than 90%?

A. 6
B. 7
C. 9
D. 10
E. 11

I will see what is the quickest way to solve it then I will provide the explanation

The following approach might be the easiest one and less error prone.

We need on-time departure rate to be higher than 9/10, so it should be at least 10/11, which means that 10 out of 11 flights must depart on time. Since for now 3 out of 4 flights departed on time then 10-3=7 subsequent flights need to depart on-time.

one flight was already late & 3 flight were on time departure so now to have a on-time departure rate to be higher than 9/10,

3 on time + 6 yet to be on time = 9 on time. total of 10, 9 on time but it not higher than 9/10 so so we went for 7 by considering 10/11?

Yes, you can say so.
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Re: The first flight out of Phoenix airport had a late departure [#permalink]  19 Sep 2013, 09:25
how did you calculate 10/11?
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Re: The first flight out of Phoenix airport had a late departure [#permalink]  26 Dec 2013, 09:46
manalq8 wrote:
The first flight out of Phoenix airport had a late departure. If the next three flights departed on-time, how many subsequent flights need to depart from Phoenix on-time, for the airport's on-time departure rate to be higher than 90%?

A. 6
B. 7
C. 9
D. 10
E. 11

I will see what is the quickest way to solve it then I will provide the explanation

3+x/4+x >9/10
x>6

Hence B is our best choice

Cheers!
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Re: The first flight out of Phoenix airport had a late departure [#permalink]  26 Dec 2013, 10:21
theGame001 wrote:
how did you calculate 10/11?

here is my approach to this question.
Let the total number of flights be 'x'. On time departure rate should be more than 90%.
it is said that 1 flight is late.... so out of x flights is 1 flight is late remaining (x-1) flights will be on-time...
so $$\frac{(x-1)}{x}*100=90$$
solving for x we get 10...... but remember we want on-time departure rate to be more than 90%, not exactly 90%..... so minimum number of flights to take off is 11.....
since first one is late and remaining 3 left so the next 7 flights should also be on-time..... So answer is 7...
Re: The first flight out of Phoenix airport had a late departure   [#permalink] 26 Dec 2013, 10:21
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