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# Mary and three other students took a math test. Each of their scores w

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VP
Joined: 19 Oct 2018
Posts: 1150
Location: India
Mary and three other students took a math test. Each of their scores w  [#permalink]

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16 May 2019, 03:20
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95% (hard)

Question Stats:

46% (02:27) correct 54% (02:17) wrong based on 134 sessions

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Mary and three other students took a math test. Each of their scores was a non-negative integer. The teacher announced that the average score (of the 4 students) was 20. Mary immediately knew that all of the other three students scored below average. What is the minimum score that Mary could have gotten to be certain of the above situation?

A. 40
B. 41
C. 60
D. 61
E. 80
Manager
Joined: 01 Oct 2018
Posts: 112
Re: Mary and three other students took a math test. Each of their scores w  [#permalink]

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12 Jun 2019, 15:07
4
1
Karmesh wrote:
nick1816 wrote:
Mary immediately knew that all of the other three students scored below average, hence Mary score must be greater than 60.

The minimum score that Mary could have gotten to be certain of the above situation=61( Maximum score that any other student can get is 19 which is smaller than the average)

TIA

1. We know, that average is 20.
2. Mary give information, that no one of other 3 student don't get more than 20

Let's say that we have Mary, student a, student b and student c

THE ONLY VARIANT , when we are sure , that no one of students a,b and c get more than 20 each is when the SUM of
their results is no more than 20

a + b + c < 20

Why is it so?

Let's imagine variant:
a+b+c = 30
So, 2 possibilities exist:
1. variant
Student a get 10
b get 10
c get 10
10+10+10=30
2. variant
a get 28
b get 1
c get 1
28+1+1=30

As we know Mary dont know results of 3 other students, but she KNOW that
NO ONE of them get result more than 20.
So, we have the only variant when a + b + c < 20

Next we calculate Mary result:
We know that
a+b+c = 19

(19+M)/4 = 20

M = 61

Answ D
##### General Discussion
VP
Joined: 19 Oct 2018
Posts: 1150
Location: India
Re: Mary and three other students took a math test. Each of their scores w  [#permalink]

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16 May 2019, 20:20
Mary immediately knew that all of the other three students scored below average, hence Mary score must be greater than 60.

The minimum score that Mary could have gotten to be certain of the above situation=61( Maximum score that any other student can get is 19 which is smaller than the average)
Manager
Joined: 18 Apr 2019
Posts: 87
Location: India
GMAT 1: 720 Q48 V40
GPA: 4
Re: Mary and three other students took a math test. Each of their scores w  [#permalink]

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11 Jun 2019, 07:01
nick1816 wrote:
Mary immediately knew that all of the other three students scored below average, hence Mary score must be greater than 60.

The minimum score that Mary could have gotten to be certain of the above situation=61( Maximum score that any other student can get is 19 which is smaller than the average)

TIA
Intern
Joined: 29 May 2019
Posts: 2
Re: Mary and three other students took a math test. Each of their scores w  [#permalink]

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14 Aug 2019, 08:06
Hi, I have a question.

The question says 3 students scored below average. What if all the 3 students scored the same marks? In which case the answer could be : 80 - 13*3 = 41. 41 is also an option.
GMAT Tutor
Joined: 24 Jun 2008
Posts: 1829
Re: Mary and three other students took a math test. Each of their scores w  [#permalink]

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14 Aug 2019, 08:38
The four scores sum to 80. Mary is completely sure, seeing her score, that the other three scores were all below average. So Mary is completely sure that no one else got a score of 20 or higher. If Mary only got a 60, then it would be possible that the four scores were 60, 20, 0 and 0, and Mary could not be sure that the three other scores were below average (since 20 would be exactly equal to the average). But if Mary gets a 61 or higher, then no one else could have a score higher than 19, and all three of the other scores must be below average. So 61 is the answer.
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Intern
Joined: 18 Jul 2019
Posts: 1
Mary and three other students took a math test. Each of their scores w  [#permalink]

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14 Aug 2019, 08:58
Since the worst possible case can be 19,0,0, the minimum sum of the possible scores is 19
Therefore Marry should score at least 80-19= 61
Intern
Joined: 29 May 2019
Posts: 2
Re: Mary and three other students took a math test. Each of their scores w  [#permalink]

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14 Aug 2019, 09:11
IanStewart wrote:
The four scores sum to 80. Mary is completely sure, seeing her score, that the other three scores were all below average. So Mary is completely sure that no one else got a score of 20 or higher. If Mary only got a 60, then it would be possible that the four scores were 60, 20, 0 and 0, and Mary could not be sure that the three other scores were below average (since 20 would be exactly equal to the average). But if Mary gets a 61 or higher, then no one else could have a score higher than 19, and all three of the other scores must be below average. So 61 is the answer.

I am still not clear about this point. Our aim is to find the lowest possible score for mary. If i were to test from the given answer options then in the case of b = 41

we would get that the rest of the 3 students have cumulatively scored 39 marks. In the simplest case, 3 three could have scored 13 each which is below the average score OR if I were to maximise then one student could have student could have scored 19 and the other two could be any score that adds up to 20, in which case their individual scores would still be below the average.

So the lowest score mary could've gotten can be 41, right?
GMAT Tutor
Joined: 24 Jun 2008
Posts: 1829
Re: Mary and three other students took a math test. Each of their scores w  [#permalink]

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14 Aug 2019, 10:44
2
abhiroopm wrote:
we would get that the rest of the 3 students have cumulatively scored 39 marks. In the simplest case, 3 three could have scored 13 each which is below the average score OR if I were to maximise then one student could have student could have scored 19

If Mary gets 41, then if you were to maximize the second-highest score, that second highest score could be 39, and the remaining two scores could be zero. It seems you're assuming in advance that the remaining three students were below-average (that's the only reason the maximum second-best score would be 19), but then you're "begging the question", in logical terms -- assuming the answer to the question before you answer it.

Mary needs to be absolutely certain, upon seeing her score, that no other student could have a score of 20 or higher. Since some scores can be 0, the only way Mary can be certain of that is if she knows the sum of the three other scores is 19 or less.
_________________
GMAT Tutor in Toronto

If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com
Re: Mary and three other students took a math test. Each of their scores w   [#permalink] 14 Aug 2019, 10:44
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