The sum of the even numbers between 1 and n is 85*86, where n is an odd number, then n=? Pick one
A) 163
B) 171
C) 179
D) 185
E) 191

Rationale: "let n - 1 = 2 a . 2 + 4 + . . . + 2 a = 2 * ( 1 + 2 + . . . + a ) = 2 * ( a ) ( a + 1 ) / 2 = ( a ) ( a + 1 ) = 85 * 86 then a = 85 and n = 171 . the answer is b ."
Answer: B