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Worked solution

Two throws: ordered outcomes and conditioning

In a game, an ordinary fair die is thrown twice and the number that appears on each throw is noted. Find the probability that

(i) both numbers are ‘6’,

(ii) neither number is a ‘5’,

(iii) one number is ‘6’ and the other is ‘5’,

(iv) the sum of the numbers is 6,

(v) the first number is less than the second given that the sum of the two numbers is 6.

CJC · Discussion · Question 1

Answer

(i) 1/361/36; (ii) 25/3625/36; (iii) 1/181/18; (iv) 5/365/36; (v) 2/52/5.

Use an ordered sample space

The first throw labels columns and the second labels rows. For independent throws all 36 ordered pairs are equally likely. The full sum table is:

Second / first123456
1234567
2345678
3456789
45678910
567891011
6789101112
How did this part go?

(i) Both six

Method 1 counts the single cell (6,6)(6,6): 1/361/36. Method 2 multiplies independent probabilities: (1/6)(1/6)=1/36(1/6)(1/6)=1/36.

How did this part go?

(ii) Neither five

Method 1 excludes the fifth row and column, leaving 25 cells. Method 2 gives (5/6)(5/6)=25/36(5/6)(5/6)=25/36.

How did this part go?

(iii) Five and six in either order

Method 1 counts (5,6)(5,6) and (6,5)(6,5): 2/36=1/182/36=1/18. Method 2 counts one specified order, then multiplies by 2!2!: (1/6)(1/6)×2=1/18(1/6)(1/6)\times2=1/18.

How did this part go?

(iv) Sum six

The five pairs are (1,5),(2,4),(3,3),(4,2),(5,1)(1,5),(2,4),(3,3),(4,2),(5,1), so the probability is 5/365/36.

How did this part go?

(v) Method 1: conditional probability

The pairs with sum six and first throw smaller are (1,5),(2,4)(1,5),(2,4). Thus

P(first<second∣sum=6)=2/365/36=25.\begin{aligned}&P(\text{first}<\text{second}\mid\text{sum}=6)\\ & =\frac{2/36}{5/36}\\ & =\frac25.\end{aligned}

How did this part go?

(v) Method 2: reduced sample space

Restrict to the five equally likely sum-six outcomes. Exactly two satisfy first<second, giving 2/52/5. The six reconstructed grid panels retain both conditional methods and the source orientation.

How did this part go?

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Columns first throw, rows second throw, each 1 to 6. Panels highlight both six; neither five; one five/one six; five sum-six cells; two favourable conditional cells; and a reduced five-cell sample space with two favourable cells. All sum-table values are shown in the last three panels. Blue encodes the event; grey marks other eligible reduced-space outcomes.
Columns first throw, rows second throw, each 1 to 6. Panels highlight both six; neither five; one five/one six; five sum-six cells; two favourable conditional cells; and a reduced five-cell sample space with two favourable cells. All sum-table values are shown in the last three panels. Blue encodes the event; grey marks other eligible reduced-space outcomes.
Columns first throw, rows second throw, each 1 to 6. Panels highlight both six; neither five; one five/one six; five sum-six cells; two favourable conditional cells; and a reduced five-cell sample space with two favourable cells. All sum-table values are shown in the last three panels. Blue encodes the event; grey marks other eligible reduced-space outcomes.

Common mistakes

After conditioning on a sum of six, divide by five eligible outcomes, not by 36.

Solution notes

All five asks, both printed methods where supplied, full sum table and six shaded-grid relationships retained. Independence is the source model for successive fair throws.

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