In a race where 10 cars are running, the chance that car X will win is 1/7, that Y will win is 1/3 and that Z will win is 1/5. Assuming that a dead heat is impossible, find the chance that one of them will win.  Pick:
A) 1 / 140  B) 1 / 180  C) 71 / 105  D) 21 / 113  E) 57 / 120

Rationale: "required probability = p ( x ) + p ( y ) + p ( z ) ( all the events are mutually exclusive ) . = 1 / 7 + 1 / 3 + 1 / 5 = 71 / 105 answer : c"
Answer: C