Q. In a tournament, 28 games were played when each team plays one game. Then how many teams participated in this tournament?
(A) 7
(B) 8
(C) 9
(D) 10
(E) 11

Here each person plays n-1 games so that all n players play same way. But n(n-1) gives playing 2 times with each person, hence we divide  n(n-1) by2
n(n-1) / 2 = 28
n(n-1) = 56 = 8 x 7
There fore n = 8