math
posted by M .
The 20 leaves of a 40page book are numbered 1 through 40. The first and last leaves are ripped out. What is the sum of the remaining page numbers?

So what is the sum of 3+4+5+...37+38 ?
It is an arithemetic sum, first term 3, d=1
http://en.wikipedia.org/wiki/Arithmetic_progression
n= 36 terms
Sum= n/2 (2a+(n1)d) 
I got 738

738
Using the formula of an Arithmetic Progression, we can solve the answer.
Since 1+2+3+4+...+37+38+39+40 = 41*20 = 820,
the answer is 820403921 = 738.
(If there are N pages in a book, there are N/2 leaves)
Hope I helped!
Milowen
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