In a race where 15 cars are running, the chance that car X will win is 1/4, that Y will win is 1/8 and that Z will win is 1/12. Assuming that a dead heat is impossible, find the chance that one of them will win.  Choose the correct choice from the following answers:
A) 1 / 140
B) 1 / 180
C) 11 / 24
D) 10 / 15
E) 57 / 120

Rationale: "required probability = p ( x ) + p ( y ) + p ( z ) ( all the events are mutually exclusive ) . = 1 / 4 + 1 / 8 + 1 / 12 = 11 / 24 answer : c"
Answer: C