VOLUNTEER TRAINING FOR DISADVANTAGED IN MATHS – QUESTION 15 : A motel cleaner has 8 master keys at home to open all the rooms in a large motel. Each room can be opened by just 1 of these keys. If 40 % of the rooms are left unlocked, what is the probability that the cleaner can get into a specific room if he selects 3 keys at random before leaving home to go to work? (A) 5 / 8, (B) 5 / 16, (C) 31 / 40, (D) 3 / 20, (E) 19 / 40.
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VOLUNTEER TRAINING FOR DISADVANTAGED IN MATHS – ANSWER 15 : A. Probability = 4 / 10 + (6 / 10) (3 / 8) = 5 / 8
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