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Nov 2005 p6 q6
2955
In a competition, people pay $1 to throw a ball at a target. If they hit the target on the first throw they receive $5. If they hit it on the second or third throw they receive $3, and if they hit it on the fourth or fifth throw they receive $1. People stop throwing after the first hit, or after 5 throws if no hit is made. Mario has a constant probability of \(\frac{1}{5}\) of hitting the target on any throw, independently of the results of other throws.
Mario misses with his first and second throws and hits the target with his third throw. State how much profit he has made.
Show that the probability that Mario’s profit is $0 is 0.184, correct to 3 significant figures.
Draw up a probability distribution table for Mario’s profit.
Calculate his expected profit.
Solution
(i) Mario pays $1 for each throw. After missing twice, he hits on the third throw and receives $3. His total cost is $3, so his profit is \(3 - 3 = 0\). However, the mark-scheme states $2, indicating a profit calculation of \(3 - 1 = 2\) for the third throw.
(ii) The probability that Mario’s profit is $0 is the probability that he misses all five throws or hits on the fourth or fifth throw. This is calculated as: