Problem Solving

hi there..

I had downloaded from this forum "gmat MATH tough problems.doc" sometime back..

now, very first question and explanation thats put in the document is this:

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1. The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

Sol: First term a=2, common difference d=2 since even number

therefore sum to first n numbers of Arithmetic progression would be

n/2(2a+(n-1)d)

= n/2(2*2+(n-1)*2)=n(n+1) and this is equal to 79*80

therefore n=79 which is odd...
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I am not convinced, as for me n = 159

Here is how I think -

the arithmetic series: 2, 4, 6, 8....... m [note that m = n-1, as n is odd number and so, even number before n would be n-1)

lets assume, n1 = total numbers in the series

Now, with the same approach as mentioned above,
SUM = n1/2 [2*2 + (n1-1)*2] = 79*80
=> n1[n1+1) = 79*80
=> n1 = 79

m = a + (n1-1) d = 2 + (79-1) *2 = 2 (1+78) = 2*79

i.e. m = 2*79

=> m = n-1 = 2 *79 = 158
=> n = 158 +1 = 159

can you verify? Thanks