# Probability for Rolling Two Dice

Probability for rolling two dice with the six sided dots

such as 1, 2, 3, 4, 5 and 6 dots in each die.

When two dice are thrown simultaneously, thus number of event can be 6^{2} = 36 because each die has 1 to 6 number on its faces. Then the possible outcomes are shown in the below table.

Probability – Sample space for two dice (outcomes):

**Note:**

(i) The outcomes (1, 1), (2, 2), (3, 3), (4, 4), (5, 5) and (6, 6) are called doublets.

(ii) The pair (1, 2) and (2, 1) are different outcomes.

Worked-out problems involving probability for rolling two dice:

**1.** Two dice are rolled. Let A, B, C be the events of getting a sum of 2, a sum of 3 and a sum of 4 respectively. Then, show that

(i) A is a simple event

(ii) B and C are compound events

(iii) A and B are mutually exclusive

**Solution:**

Clearly, we have

A = {(1, 1)}, B = {(1, 2), (2, 1)} and C = {(1, 3), (3, 1), (2, 2)}.

(i) Since A consists of a single sample point, it is a simple event.

(ii) Since both B and C contain more than one sample point, each one of them is a compound event.

(iii) Since A ∩ B = ∅, A and B are mutually exclusive.

**2.** Two dice are rolled. A is the event that the sum of the numbers shown on the two dice is 5, and B is the event that at least one of the dice shows up a 3.

Are the two events (i) mutually exclusive, (ii) exhaustive? Give arguments in support of your answer.

**Solution: **

When two dice are rolled, we have n(S) = (6 × 6) = 36.

Now, A = {(1, 4), (2, 3), (4, 1), (3, 2)}, and

B = {(3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (3, 6), (1,3), (2, 3), (4, 3), (5, 3), (6, 3)}

(i) A ∩ B = {(2, 3), (3, 2)} ≠ ∅.

Hence, A and B are not mutually exclusive.

(ii) Also, A ∪ B ≠ S.

Therefore, A and B are not exhaustive events.

More examples related to the questions on the probabilities for throwing two dice.

**3.** Two dice are thrown simultaneously. Find the probability of:

(i) getting six as a product

(ii) getting sum ≤ 3

(iii) getting sum ≤ 10

(iv) getting a doublet

(v) getting a sum of 8

(vi) getting sum divisible by 5

(vii) getting sum of atleast 11

(viii) getting a multiple of 3 as the sum

(ix) getting a total of atleast 10

(x) getting an even number as the sum

(xi) getting a prime number as the sum

(xii) getting a doublet of even numbers

(xiii) getting a multiple of 2 on one die and a multiple of 3 on the other die

**Solution:**

Two different dice are thrown simultaneously being number 1, 2, 3, 4, 5 and 6 on their faces. We know that in a single thrown of two different dice, the total number of possible outcomes is (6 × 6) = 36.

**(i) getting six as a product:**

Let E_{1} = event of getting six as a product. The number whose product is six will be E_{1} = [(1, 6), (2, 3), (3, 2), (6, 1)] = 4

Therefore, probability of

getting ‘six as a product’

Number of favorable outcomes

P(E_{1}) =

Total number of possible outcome

= 4/36

= 1/9

**(ii) getting sum ≤**** 3:**

Let E_{2} = event of getting sum ≤ 3. The number whose sum ≤ 3 will be E_{2} = [(1, 1), (1, 2), (2, 1)] = 3

Therefore, probability of

getting ‘sum ≤ 3’

Number of favorable outcomes

P(E_{2}) =

Total number of possible outcome

= 3/36

= 1/12

**(iii) getting sum ≤**** 10:**

Let E_{3} = event of getting sum ≤ 10. The number whose sum ≤ 10 will be E_{3} =

[(1,

1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6),

(2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (2,

6),

(3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (3,

6),

(4, 1), (4, 2), (4, 3), (4, 4), (4, 5), (4,

6)

(5, 1), (5, 2), (5, 3), (5, 4), (5, 5),

(6, 1), (6, 2), (6, 3), (6, 4)] = 33

Therefore, probability of

getting ‘sum ≤ 10’

Number of favorable outcomes

P(E_{3}) =

Total number of possible outcome

= 33/36

= 11/12

**(iv)
getting a doublet:**

Let E

_{4}= event of getting a doublet. The number which doublet will be E

_{4}= [(1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)] = 6

Therefore, probability of

getting ‘a doublet’

Number of favorable outcomes

P(E_{4}) =

Total number of possible outcome

= 6/36

= 1/6

**(v)
getting a sum of 8:**

Let E_{5} = event of getting a sum of 8. The number which is a sum of 8 will be E_{5} = [(2, 6), (3, 5), (4, 4), (5, 3), (6, 2)] = 5

Therefore, probability of

getting ‘a sum of 8’

Number of favorable outcomes

P(E_{5}) =

Total number of possible outcome

= 5/36

**(vi)
getting sum divisible by 5:**

Let E_{6} = event of getting sum divisible by 5. The number whose sum divisible by 5 will be E_{6} = [(1, 4), (2, 3), (3, 2), (4, 1), (4, 6), (5, 5), (6, 4)] = 7

Therefore, probability of

getting ‘sum divisible by 5’

Number of favorable outcomes

P(E_{6}) =

Total number of possible outcome

= 7/36

**(vii)
getting sum of atleast 11:**

Let E_{7} = event of getting sum of atleast 11. The events of the sum of atleast 11 will be E_{7} = [(5, 6), (6, 5), (6, 6)] = 3

Therefore, probability of

getting ‘sum of atleast 11’

Number of favorable outcomes

P(E_{7}) =

Total number of possible outcome

= 3/36

= 1/12

**(viii) getting a
multiple of 3 as the sum:**

Let E_{8} = event of getting a multiple of 3 as the sum. The events of a multiple of 3 as the sum will be E_{8} = [(1, 2), (1, 5), (2, 1), (2, 4), (3, 3), (3, 6), (4, 2), (4, 5), (5, 1), (5, 4), (6, 3) (6, 6)] = 12

Therefore, probability of

getting ‘a multiple of 3 as the sum’

Number of favorable outcomes

P(E_{8}) =

Total number of possible outcome

= 12/36

= 1/3

**(ix) getting a total
of atleast 10:**

Let E_{9} = event of getting a total of atleast 10. The events of a total of atleast 10 will be E_{9} = [(4, 6), (5, 5), (5, 6), (6, 4), (6, 5), (6, 6)] = 6

Therefore, probability of

getting ‘a total of atleast 10’

Number of favorable outcomes

P(E_{9}) =

Total number of possible outcome

= 6/36

= 1/6

**(x) getting an even
number as the sum:**

Let E_{10} = event of getting an even number as the sum. The events of an even number as the sum will be E_{10} = [(1, 1), (1, 3), (1, 5), (2, 2), (2, 4), (2, 6), (3, 3), (3, 1), (3, 5), (4, 4), (4, 2), (4, 6), (5, 1), (5, 3), (5, 5), (6, 2), (6, 4), (6, 6)] = 18

Therefore, probability of

getting ‘an even number as the sum

Number of favorable outcomes

P(E_{10}) =

Total number of possible outcome

= 18/36

= 1/2

**(xi) getting a prime
number as the sum:**

Let E_{11} = event of getting a prime number as the sum. The events of a prime number as the sum will be E_{11} = [(1, 1), (1, 2), (1, 4), (1, 6), (2, 1), (2, 3), (2, 5), (3, 2), (3, 4), (4, 1), (4, 3), (5, 2), (5, 6), (6, 1), (6, 5)] = 15

Therefore, probability of

getting ‘a prime number as the sum’

Number of favorable outcomes

P(E_{11}) =

Total number of possible outcome

= 15/36

= 5/12

**(xii) getting a
doublet of even numbers:**

Let E_{12} = event of getting a doublet of even numbers. The events of a doublet of even numbers will be E_{12} = [(2, 2), (4, 4), (6, 6)] = 3

Therefore, probability of

getting ‘a doublet of even numbers’

Number of favorable outcomes

P(E_{12}) =

Total number of possible outcome

= 3/36

= 1/12

**(xiii) getting a
multiple of 2 on one die and a multiple of 3 on the other die:**

Let E_{13} = event of getting a multiple of 2 on one die and a multiple of 3 on the other die. The events of a multiple of 2 on one die and a multiple of 3 on the other die will be E_{13} = [(2, 3), (2, 6), (3, 2), (3, 4), (3, 6), (4, 3), (4, 6), (6, 2), (6, 3), (6, 4), (6, 6)] = 11

Therefore, probability of

getting ‘a multiple of 2 on one die and a multiple of 3 on the other die’

Number of favorable outcomes

P(E_{13}) =

Total number of possible outcome

= 11/36

**4.** Two

dice are thrown. Find (i) the odds in favour of getting the sum 5, and (ii) the

odds against getting the sum 6.

**Solution:**

We know that in a single thrown of two die, the total number

of possible outcomes is (6 × 6) = 36.

Let S be the sample space. Then,

n(S) = 36.

**(i) the odds in favour of getting the sum 5:**

Let E_{1} be the event of getting the sum 5. Then,

E_{1} = {(1, 4), (2, 3), (3, 2), (4, 1)}

⇒ P(E_{1}) = 4

Therefore, P(E_{1}) = n(E_{1})/n(S) = 4/36 = 1/9

⇒ odds in favour of E_{1} = P(E_{1})/[1 – P(E_{1})] = (1/9)/(1 – 1/9) = 1/8.

**(ii) the odds against getting the sum 6:**

Let E_{2} be the event of getting the sum 6. Then,

E_{2} = {(1, 5), (2, 4), (3, 3), (4, 2), (5, 1)}

⇒ P(E_{2}) = 5

Therefore, P(E_{2}) = n(E_{2})/n(S) = 5/36

⇒ odds against E_{2} = [1 – P(E_{2})]/P(E_{2}) = (1 – 5/36)/(5/36) = 31/5.

**5.** Two dice, one blue and one orange, are rolled simultaneously. Find the probability of getting

(i) equal numbers on both

(ii) two numbers appearing on them whose sum is 9.

**Solution:**

The possible outcomes are

(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (1, 6),

(2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (2, 6),

(3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (3, 6),

(4, 1), (4, 2), (4, 3), (4, 4), (4, 5), (4, 6)

(5, 1), (5, 2), (5, 3), (5, 4), (5, 5), (5, 6)

(6, 1), (6, 2), (6, 3), (6, 4), (6, 5), (6, 6)

Therefore, total number of possible outcomes = 36.

(i) Number of favourable outcomes for the event E

= number of outcomes having equal numbers on both dice

= 6 [namely, (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), (6, 6)].

So, by definition, P(E) = 636

636

= 16

16

(ii) Number of favourable outcomes for the event F

= Number of outcomes in which two numbers appearing on them have the sum 9

= 4 [namely, (3, 6), (4, 5), (5, 4), (3, 6)].

Thus, by definition, P(F) = 436

436

= 19

19.These examples will help

us to solve different types of problems based on probability for rolling

two dice.