KarishmaB , please check whether I understood it correctly. You are just brilliant . I wish I were half as smart as you are. No amount of kudos should be enough for you.
Total excess = 75
Total deficit = 125
Net difference (deficit) which is spread across all the 10 terms= 125 - 75 = 50 .
Net difference = 50 for all the 10 terms.
Hence average difference = 50/10 =5
Initially we took the mean of 10 terms = 75.
Average difference to it =5
Hence, net mean for all the 10 terms = 75 - 50/10 = 70
MartyMurrayKarishmaB wrote:
sayan640 wrote:
KarishmaB MartyMurray Any faster way to approach this question ? Finding out the mean takes a lot of time and is also risky in view of silly mistakes. Can you please help with a quicker approach ?
gmatmania17 wrote:
40, 45, 50, 55, 60, 75, 75, 100, 100, 100.
The list above shows the scores of 10 schoolchildren on a certain test. If the standard deviation of the 10 scores is 22.4, rounded to the nearest tenth, how many of the scores are more than 1 standard deviation below the mean of the 10 scores?
A. One
B. Two
C. Three
D. Four
E. Five
I arrived to the solution but i would like to find a quickest way..I computed the average for the scores from 40 to 60 which is the number in the middle because they are evenly spaced. Then I multiply 50*5added 2*75 and 3*100 and divided everything by 10 obtaining the average which is 70.
Then I subtracted 22.4 from 70 and I found : 47.6. Then I counted the scores that are less than 47.6 that are 2 (40,45) and i arrived at the answer..
Thank you in advance!
The solution given by Bunuel is the fastest approach.
To get the mean, I would use deviations. I would assume mean to be 75 and find the deviations.
Excess = 3*25 = 75
Deficit = 15 + 20 + 25 + 30 + 35 = (15 + 35)/2 * 5 = 125
Hence mean = 75 - 50/10 = 701 SD below mean means value should be below 70 - 22.4 = 47.6
There are 2 such values: 40 & 45
Answer (B)
Deviations is discussed in this blog post:
https://anaprep.com/arithmetic-usefulness-of-deviations/