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patrakakali79
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Average

by patrakakali79 Fri Nov 01, 2013 3:30 pm

The Average(arithmetic mean) of 6 consecutive 1 integers is 18.5.What is the avg. of the 1st 5 of these integers?

(i) 12.5
(ii) 15
(iii) 16
(iv)17.5
(v) 18

Source : GRE Kaplan Math workbook
tommywallach
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Re: Average

by tommywallach Mon Nov 04, 2013 2:22 pm

Hey Patra,

So we can just set the first integer to x, and use the average formula

(x + (x + 1) + (x + 2) + (x + 3) + (x + 4) + (x + 5)) / 6 = 18.5

Now multiply both sides by 6, and add up all the x stuff:

6x + 15 = 111
6x = 96
x = 16

OR, you could know that the median = mean in any evenly spaced set, so we know 18.5 is the median, which means the two numbers on either side are 18 and 19, and then we can just count downwards to 16.

Hope that helps!

-t
patrakakali79
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Re: Average

by patrakakali79 Tue Nov 12, 2013 1:37 pm

got it!

Thanks
tommywallach
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Re: Average

by tommywallach Tue Nov 12, 2013 3:04 pm

Glad to help!

-t
lorelondon2013
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Re: Average

by lorelondon2013 Tue Nov 26, 2013 2:01 am

Does this mean that the average of the first 5 integers is actually 18?
tommywallach
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Re: Average

by tommywallach Tue Nov 26, 2013 7:47 pm

It does!

(16 + 17 + 18 + 19 + 20) / 5 = 18

(Also, the median = mean for any set of evenly spaced numbers, so we can see that the mean is 18.)

-t