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The list shown consist of the times, in seconds, that it [#permalink]
21 Aug 2007, 16:27

00:00

A

B

C

D

E

Difficulty:

5% (low)

Question Stats:

30% (02:14) correct
69% (01:02) wrong based on 13 sessions

70,75, 80,85,90,105,105,130,130,130

The list consists of the times in seconds that it took each of the 10 school children to run a distance of 400 mts . If the standard deviation of the 10 running times is 22.4, 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.

Re: PS- Standard Deviations [#permalink]
21 Aug 2007, 16:39

forgmat wrote:

70,75, 80,85,90,105,105,130,130,130

The list consists of the times in seconds that it took each of the 10 school children to run a distance of 400 mts . If the standard deviation of the 10 running times is 22.4, 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. 1 b.2 c.3 d.4 e.5

I think the key here is to know what "1 standard deviation" means.
1 standard deviation = 1 * 22.4 = 22.4
Now, find the average:
Avg = (70+75+80+85+90+105+105+130+130+130)/10 = 1000/10 = 100
So rage should be 100 +/- 22.4 = 77.6 and 122.4
Looks like E is the answer.

1) Median is (90+105)/2 = 97.5
2) St. deviation: 97.5 +/- 22.4 = 75,1 from the left, and 119,9 from the right.
3) From the left we have 2 child (70 and 75), and from the right - 3 child (130, 130 and 130)

Ans. E

P.S> I can't see how the answ. can be 2, if assw. is 2 - it logically could be only 70 and 75, but if you add 2 st. dev. = 44.8 to 80, you'll get 124.8 - so 3*130 still do not fit in this pattern %)

Last edited by Whatever on 21 Aug 2007, 17:50, edited 1 time in total.

sorry just found a link where this question was posted earlier on gmat club. no the answer is 2

I think I see the point here. Although five people are in the "range", three are above, and two are below. So the answer is 2, I guess that's what the question is asking for.

Re: 70,75, 80,85,90,105,105,130,130,130 The list consists of [#permalink]
20 Jan 2012, 01:52

1

This post received KUDOS

Expert's post

ruturajp wrote:

how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times. I din get the question can any1 explain

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

"How many of the 10 running times are more than one SD below the mean" means how many data points from given 10 are less than mean-1SD.

We are given that SD=22.4, so we should find mean --> mean=100 --> there are only 2 data points below 100-22.4=77.6, namely 70 and 75.