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June 2019 p61 q6
2904
At a funfair, Amy pays $1 for two attempts to make a bell ring by shooting at it with a water pistol.
If she makes the bell ring on her first attempt, she receives $3 and stops playing. This means that overall she has gained $2.
If she makes the bell ring on her second attempt, she receives $1.50 and stops playing. This means that overall she has gained $0.50.
If she does not make the bell ring in the two attempts, she has lost her original $1.
The probability that Amy makes the bell ring on any attempt is 0.2, independently of other attempts.
Show that the probability that Amy loses her original $1 is 0.64. [2]
Complete the probability distribution table for the amount that Amy gains. [4]
Calculate Amy’s expected gain. [1]
Solution
(i) The probability that Amy loses her original $1 is the probability that she fails both attempts. The probability of failing one attempt is given by:
\(P(F) = 1 - 0.2 = 0.8\)
Therefore, the probability of failing both attempts is: