A set of football matches is to be organized in a "round-robin" fashion, i.e., every
participating team plays a match against every other team once and only once. If 15
matches are totally played, how many teams participated?	Choose the correct choice from the following answers:
A) 4 B) 5 C) 6 D) 7 E) 8

Rationale: 2 ways to solve this problem . . summation ( x ) = n ( n - 1 ) / 2 n ( n - 1 ) / 2 = 15 ; n ^ 2 - n - 30 = 0 ans : 6 answer : c
Answer: C