Check my math on these two probabilties, please

[twistedbeats]

Warning, i dropped prob and stat.

1)if the odds of an event happening are 1 in 8, what are the odds of it not happening in 13 tries:

(7/8)^ 13 = .17624

2) what are the odds of the same event happening exactly two times in 18 trials

(1/8)^2 * (7/8)^16 * 18 choose 2= .282254