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Difficult Math Question #1

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by nisagl750 » Tue Nov 27, 2012 4:32 am
ttwang56 wrote:shouldn't n = 157?

if n = 159 then the way to check would be 158+2 = 160. 160 would be able to added up all the with 4+156, etc. 160 doesn't go into the 6320 (78*80) evenly. However, if n = 157, then 2+156= 158. 158 goes evenly into 6320 with a solution of 40.

why can't u solve it by factoring 80 to being 2 x 40 and multiplying the 2 to the 79?

don't know if i explained this clearly or not.
n=159

question says....SUM of all even numbers from 1 to n-1 is 79*80

so.. if n=159, n-1 = 158

We need to add..... 2+4+6+.....158
By A.P. formula

Sum = n/2(first term + last term), Here n = number of terms, not the n from our question.(which is 79)

Sum =79*(2+158) / 2
=79*160/2
=79*80

I hope you got it.....

My ans
n is 159