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The list shown consist of the times, in seconds, that it
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70, 75, 80, 85, 90, 105, 105, 130, 130, 130 The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 on of meters. If the standard devastation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times? A. one B. two C. three D. four E. five
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Originally posted by raghavs on 28 Mar 2010, 09:42.
Last edited by Bunuel on 14 Oct 2014, 06:00, edited 3 times in total.
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Re: The list shown consist of the times, in seconds, that it
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23 Jan 2012, 03:01




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Re: The list shown consist of the times, in seconds, that it
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28 Mar 2010, 11:13
raghavs wrote: standard deviation..first question stumped me the mean of the 10 running times is 100 sec. we need to find how many of the recorded times are less than 1 SD below the mean i.e how many took less than 100 sec  1SD = 100 sec  22.4 sec = 77.6sec. The times are 70 and 75. hence 2



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28 Mar 2010, 21:35
raghavs wrote: standard deviation..first question stumped me mean is 99 SD is 22.4 One SD block away = 9922.4 = 76.6 so there are two numbers less than mean which are One SD block away 70 & 75 hence TWO.



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Re: The list shown consist of the times, in seconds, that it
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23 Jan 2012, 06:57
first add the values and devide by 10 to get the mean of 100. then butract 22.4 from the 100.....> 10022.4=76.6 check how many numbers in the list are less than 76.6! there are two numbers hope this helps
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Re: The list shown consist of the times, in seconds, that it
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03 Jun 2013, 02:09



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03 Jun 2013, 04:15
The first step here is to calculate the mean :
(70+75+80+85+90+105+105+130+130+130)/10 = 100
SD = 22.4,
one SD below the mean = (100 – 22.4) = 77.6
Only 2 values 70 and 75 are less than 77.6, hence B is the answer



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03 Jun 2013, 15:23
To find the mean, take a wild stab and pick a number within the range provided by the given numbers. Now, find the delta between that number and each number in the set. So now you get the overall delta from your "assumed mean". Split that up equally among each number. So adding that to your "assumed mean" will give you the actual mean.
I took 100 and found the difference between it and each of the numbers in the set. Turns out that the overall delta neatly becomes 0. So 100 is the actual mean.



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04 Jun 2013, 03:39
70 75 80 85 90 105 105 130 130 130 The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B
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Re: The list shown consist of the times, in seconds, that it
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03 Sep 2013, 23:55
ziko wrote: 70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B Pardon me for my understanding, but "are <more than> <one SD below the mean> of the 10 running times? So isn't it asking values greater than 77.6 ?



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Re: The list shown consist of the times, in seconds, that it
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04 Sep 2013, 01:49
ygdrasil24 wrote: ziko wrote: 70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B Pardon me for my understanding, but "are <more than> <one SD below the mean> of the 10 running times? So isn't it asking values greater than 77.6 ? WRONG reading: How many of the 10 running times are more than one SD below the mean. So more than MeanSD. CORRECT reading: How many of the 10 running times are more than one SD below the mean. So less than MeanSD.
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Re: The list shown consist of the times, in seconds, that it
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04 Sep 2013, 02:13
Bunuel wrote: ygdrasil24 wrote: ziko wrote: 70 75 80 85 90 105 105 130 130 130
The list shown consist of the times, in seconds, that it took each of 10 school children to run a distance of 400 meter. If the SD of ten running times is 22.4 seconds, rounded to nearest tenth of second, how many of the 10 running times are more than one SD below the mean of the 10 running times? A. one B. two C. three D. four E. five
The most time consuming part in this question is to define the mean. Under exam pressure and time pressure it is very easy to make mistake. it is easier to group numbers: 130*3=390; 105*2=210; 75+85=160; 70+80=150; 90; Next stage combine results, again using more convenient ways to calculate: 390+210=600; 160+150=310; 90. 600+310+90=1000. Since there are 10 numbers the mean is 100. Questions asks to find the quantity of numbers one SD BELOW the mean, which is 10022,4=77,6. There are only two numbers below 77,6. The answer is B Pardon me for my understanding, but "are <more than> <one SD below the mean> of the 10 running times? So isn't it asking values greater than 77.6 ? WRONG reading: How many of the 10 running times are more than one SD below the mean. So more than MeanSD. CORRECT reading: How many of the 10 running times are more than one SD below the mean. So less than MeanSD. Why couldnt they have written it as How many of the 10 running times are less than one SD below the mean. This is frustrating for me



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13 Jul 2016, 17:24
Instead of wasting valuable time adding up all 10 numbers, you can spot some easy clues the GMAT leaves behind. The first 5 numbers are perfectly symmetric around 80, so you know mean of the first 5 is 80. The second half of numbers can be quickly added together 3*130 = 390 and 105*2 = 210 for a sum of 600. Divide by 5 to get 120. Since the first half has a mean of 80 and second half has a mean of 120, and both halves have equal weight (they both represent 5 integers), the average of both halves will be right in the middle at 100. From there figuring out the answer is business as usual. Sent from my SMG928T using GMAT Club Forum mobile app



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Re: The list shown consist of the times, in seconds, that it
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19 Oct 2018, 14:23
raghavs wrote: 70, 75, 80, 85, 90, 105, 105, 130, 130, 130 The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 on of meters. If the standard devastation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?
A. one B. two C. three D. four E. five The average (mean) is: (70 + 75 + 80 + 85 + 90 + 105 + 105 + 130 +130 +130)/10 = 1000/10 = 100 Thus, 1 standard deviation below the mean is 100  22.4 = 77.6, so there are 2 running times that are more than 1 standard deviation below the mean. Answer: B
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