Complete a biased-die distribution, calculate expectation and a repeated-roll probability.

Complete a biased-die distribution, calculate expectation and a repeated-roll probability.

7 marks
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In a game a player rolls a biased four-faced die. The probability of each possible score is shown below.

Score1234
Probability\(\frac15\)\(\frac25\)\(\frac1{10}\)\(x\)
  1. Find the value of x.[2]
  2. Find \(E(X)\).[3]
  3. The die is rolled twice. Find the probability of obtaining two scores of 3.[2]

Solution:

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Answer:\(E(X) = 1 \left(\frac{1}{5} \right)+2 \left(\frac{2}{5}\right)+3 \left(\frac{1}{10} \right)+4 \left(\frac{3}{10} \right)=2.5\). Two scores of 3 have probability \( \left(\frac1{10} \right)^2=0.01\).

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