Tetrahedral dice have four faces. Two fair tetrahedral dice, one red and one blue, have faces numbered 0, 1, 2, and 3 respectively. The dice are rolled, and the numbers faced down on the two dice are recorded. The random variable TT is defined as the score on the red die multiplied by the score on the blue die.

  1. (i)

    Find the probability distribution of TT.

  2. (ii)

    Find E(T)E(T) and show that Var(T)=11516\mathrm{Var}(T)=\dfrac{115}{16}. Show your workings clearly.

  3. (iii)

    Evaluate P(∣T−2μ∣>σ)P(|T-2\mu|>\sigma), where μ=E(T)\mu=E(T) and σ2=Var(T)\sigma^2=\mathrm{Var}(T).

Work through it with AI →

It won't just hand you the answer — it asks where you're stuck.How the checker works

Worked solution

6(i)

1. Construct a table of outcomes to find all possible values of $T$.

The possible scores on each die are 0,1,2,30, 1, 2, 3. There are 4×4=164 \times 4 = 16 equally likely outcomes for the pair of dice.
The product of the scores, TT, can be represented in a table of outcomes:

×012300000101232024630369\begin{array}{c|cccc} \times & 0 & 1 & 2 & 3 \\ \hline 0 & 0 & 0 & 0 & 0 \\ 1 & 0 & 1 & 2 & 3 \\ 2 & 0 & 2 & 4 & 6 \\ 3 & 0 & 3 & 6 & 9 \end{array}

2. Calculate the probability for each value of $T$.

By counting the frequencies of each possible value of TT out of the 1616 total outcomes, we have:
P(T=0)=716P(T=0) = \frac{7}{16}
P(T=1)=116P(T=1) = \frac{1}{16}
P(T=2)=216=18P(T=2) = \frac{2}{16} = \frac{1}{8}
P(T=3)=216=18P(T=3) = \frac{2}{16} = \frac{1}{8}
P(T=4)=116P(T=4) = \frac{1}{16}
P(T=6)=216=18P(T=6) = \frac{2}{16} = \frac{1}{8}
P(T=9)=116P(T=9) = \frac{1}{16}

3. Tabulate the probability distribution of $T$.

Summarising the probabilities into a formal distribution table:

t0123469P(T=t)716116181811618116\begin{array}{|c|c|c|c|c|c|c|c|} \hline t & 0 & 1 & 2 & 3 & 4 & 6 & 9 \\ \hline P(T=t) & \frac{7}{16} & \frac{1}{16} & \frac{1}{8} & \frac{1}{8} & \frac{1}{16} & \frac{1}{8} & \frac{1}{16} \\ \hline \end{array}
Answer
t0123469P(T=t)716116181811618116\begin{array}{|c|c|c|c|c|c|c|c|} \hline t & 0 & 1 & 2 & 3 & 4 & 6 & 9 \\ \hline P(T=t) & \frac{7}{16} & \frac{1}{16} & \frac{1}{8} & \frac{1}{8} & \frac{1}{16} & \frac{1}{8} & \frac{1}{16} \\ \hline \end{array}

6(ii)

1. Determine the probability distribution of $T$ (from part (i)) and calculate the expected value $E(T)$.

The possible outcomes for T=X×YT = X \times Y where X,Y∈{0,1,2,3}X, Y \in \{0, 1, 2, 3\} give the following probability distribution:
P(T=0)=716P(T=0) = \frac{7}{16}, P(T=1)=116P(T=1) = \frac{1}{16}, P(T=2)=216P(T=2) = \frac{2}{16}, P(T=3)=216P(T=3) = \frac{2}{16}, P(T=4)=116P(T=4) = \frac{1}{16}, P(T=6)=216P(T=6) = \frac{2}{16}, P(T=9)=116P(T=9) = \frac{1}{16}.

E(T)=∑tP(T=t)=0(716)+1(116)+2(216)+3(216)+4(116)+6(216)+9(116)=0+1+4+6+4+12+916=3616=94\begin{aligned} E(T) &= \sum t P(T=t) \\ &= 0\left(\frac{7}{16}\right) + 1\left(\frac{1}{16}\right) + 2\left(\frac{2}{16}\right) + 3\left(\frac{2}{16}\right) + 4\left(\frac{1}{16}\right) + 6\left(\frac{2}{16}\right) + 9\left(\frac{1}{16}\right) \\ &= \frac{0 + 1 + 4 + 6 + 4 + 12 + 9}{16} \\ &= \frac{36}{16} \\ &= \frac{9}{4} \end{aligned}

2. Calculate $E(T^2)$ and apply the variance formula $\mathrm{Var}(T) = E(T^2) - [E(T)]^2$ to show the required result.

E(T2)=∑t2P(T=t)=02(716)+12(116)+22(216)+32(216)+42(116)+62(216)+92(116)=0+1+8+18+16+72+8116=19616=494\begin{aligned} E(T^2) &= \sum t^2 P(T=t) \\ &= 0^2\left(\frac{7}{16}\right) + 1^2\left(\frac{1}{16}\right) + 2^2\left(\frac{2}{16}\right) + 3^2\left(\frac{2}{16}\right) + 4^2\left(\frac{1}{16}\right) + 6^2\left(\frac{2}{16}\right) + 9^2\left(\frac{1}{16}\right) \\ &= \frac{0 + 1 + 8 + 18 + 16 + 72 + 81}{16} \\ &= \frac{196}{16} \\ &= \frac{49}{4} \end{aligned}

Var(T)=E(T2)−[E(T)]2=494−(94)2=19616−8116=11516\begin{aligned} \mathrm{Var}(T) &= E(T^2) - [E(T)]^2 \\ &= \frac{49}{4} - \left(\frac{9}{4}\right)^2 \\ &= \frac{196}{16} - \frac{81}{16} \\ &= \frac{115}{16} \end{aligned}

Answer

E(T)=94E(T) = \frac{9}{4}

6(iii)

1. Substitute the values of $\mu$ and $\sigma$ to form an inequality for $T$.

From part (ii), we have E(T)=μ=94=2.25E(T) = \mu = \frac{9}{4} = 2.25 and Var(T)=11516\mathrm{Var}(T) = \frac{115}{16}.
Thus, σ=11516=1154≈2.681\sigma = \sqrt{\frac{115}{16}} = \frac{\sqrt{115}}{4} \approx 2.681.
The expression ∣T−2μ∣>σ|T - 2\mu| > \sigma becomes:
∣T−2(2.25)∣>2.681|T - 2(2.25)| > 2.681
∣T−4.5∣>2.681|T - 4.5| > 2.681

2. Solve the modulus inequality to find the range of valid values for $T$.

T−4.5<−2.681orT−4.5>2.681T - 4.5 < -2.681 \quad \text{or} \quad T - 4.5 > 2.681
T<1.819orT>7.181T < 1.819 \quad \text{or} \quad T > 7.181

3. Identify the values of $T$ from its probability distribution that satisfy the inequality.

The possible values of TT (the product of the scores on the two dice) are 0,1,2,3,4,6,0, 1, 2, 3, 4, 6, and 99.
The values of TT that satisfy T<1.819T < 1.819 or T>7.181T > 7.181 are 0,1,0, 1, and 99.

4. Calculate the total probability for the identified values of $T$.

P(∣T−2μ∣>σ)=P(T=0)+P(T=1)+P(T=9)\mathrm{P}(|T - 2\mu| > \sigma) = \mathrm{P}(T = 0) + \mathrm{P}(T = 1) + \mathrm{P}(T = 9)
Using the probabilities of outcomes:
=716+116+116= \frac{7}{16} + \frac{1}{16} + \frac{1}{16}
=916= \frac{9}{16}

Answer

916\frac{9}{16}

AI analysis — may contain mistakes.

Worked this one out on paper? Snap your steps and Gigle's AI math checker will circle the step where it went wrong.

More questions on this topic

  1. 1Anthony plays a game using a four-sided fair die with the numbers 1, 2, 3 and 4 printed on its sides. Anthony rolls the…
  2. 2An amateur music composer is arranging a sequence of four musical notes followed by three beats. There are 7 possible no…
  3. 3The events AA and BB are such that P(B)=13\mathrm{P}(B) = \frac{1}{3}, P(A∣B)=25\mathrm{P}(A|B) = \frac{2}{5} and P(A∪B)=35\mathrm{P}(A \cup B) = \frac{3}{5}…
  4. 4Two brothers Kai and Leo are duathlon (running and cycling) athletes who train regularly. For each day, the probability…
  5. 5A pair of siblings, Abbey and Betty, went to a book fair with six other friends. The eight of them stood in a queue in r…