Probability in a Two-Draw Lucky Dip

Probability in a Two-Draw Lucky Dip

Secondary 4

At a shopping mall, one ticket is given to every customer who spends \(\$1000\) or more. With the ticket, the customer will be invited to draw a ball from the lucky draw box containing \(35\) white and \(5\) orange balls.

If the first draw is an orange ball, the customer wins a special prize. The draw ends and the ball is not replaced.

If the first draw is a white ball, the ball is replaced in the box and the customer is invited to draw a second time.

If the second draw is an orange ball, the customer wins the special prize. The draw ends and the ball is not replaced.

If the second draw is a white ball, the ball is replaced in the box and customer walks away with nothing.

The lucky draw is over when all the \(5\) orange balls have been drawn by customers.

  1. If none of the special prizes have been won yet, find the probability that the customer with the ticket
    1. draws \(2\) white balls,
    2. wins the special prize.
  2. If only one orange ball is left, find the probability of winning the special prize.
  3. By considering your answers and the format of the lucky draw, will it take the next customer a longer/shorter/same amount of time to win the special prize?
    Briefly explain your answer.

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Answer:(a)(i) \(49/64\); (a)(ii) \(15/64\); (b) \(71/1296\). (c) Lower win probability; longer expected wait under unchanged independent participation and arrival rate, not a guaranteed individual time.

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