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In the period between 1975 and 1985, the population of each of the …ve

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In the period between 1975 and 1985, the population of each of the …ve  [#permalink]

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New post 14 Apr 2019, 14:12
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In the period between 1975 and 1985, the population of each of the five major cities in country C increased by at least 10,000 people. If the standard deviation of the populations in 1975 was 35,000 people, what was the standard deviation of the populations in 1985?

(1) The total increase in population of the five cities from 1975 to 1985 was 220,000 people.

(2) Each of the five cities increased in population by 12% in the period between 1975 and 1985.

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Re: In the period between 1975 and 1985, the population of each of the …ve  [#permalink]

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New post 14 Apr 2019, 19:31
Noshad wrote:
In the period between 1975 and 1985, the population of each of the five major cities in country C increased by at least 10,000 people. If the standard deviation of the populations in 1975 was 35,000 people, what was the standard deviation of the populations in 1985?

(1) The total increase in population of the five cities from 1975 to 1985 was 220,000 people.

(2) Each of the five cities increased in population by 12% in the period between 1975 and 1985.


Some points on SD..
1. If you add or subtract same number from all elements, the SD remains the same as the spread of numbers remains the same. It is just that the set moves up or down in similar fashion
2. If you multiply each element by same number say x, multiply the SD by |x|


Ok.. we are given SD in 1975 and we are looking for SD in 1985.
Statement I tells us that the total increase is 220,000 during this period. But we require each element to get SD
Statement II tells us that each element increased by 12%, so each element is 1.12(element).. Hence SD will be 1.12(old SD)=1.12(35,000)
Sufficient

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Re: In the period between 1975 and 1985, the population of each of the …ve   [#permalink] 14 Apr 2019, 19:31
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