n=159ttwang56 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.
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
