Two events

Two fair cubical dice are thrown at the same time and their scores added together.

In which one of the following cases are the two events not equally likely?

Event 7 : 7: Sum of scores < 7 < 7 . Event 8 : 8: Sum of scores > 7 > 7 Event 1 : 1: Sum of scores is 2 2 . Event 2 : 2: Sum of scores is 12 12 Event 5 : 5: Sum of scores is prime. Event 6 : 6: Sum of scores is not prime Event 3 : 3: Sum of scores is even. Event 4 : 4: sum of scores is odd.

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1 solution

Event 3 : 3: Sum of scores is even. Event 4 : 4: sum of scores is odd.

There are three odd and three even numbers between 1 , 2 , 3 , 4 , 5 , 6 1, 2, 3, 4, 5, 6 . Since even + even = even = odd + odd \text{even}+\text{even}=\text{even}=\text{odd}+\text{odd} , from the 6 6 = 36 6*6=36 ways, 3 3 + 3 3 = 18 3*3+3*3=18 ways are even, which is equal to 36 18 = 18 36-18=18 . So Event 3 3 and 4 4 have the same probality.

Event 1 : 1: Sum of scores is 2 : 2: . vent 2 : 2: sum of scores is 12 : 12: .

The 2 2 can only produced by 1 + 1 1+1 , and 12 12 only by 6 + 6 6+6 , so they have the same probality.

Event 5 : 5: Sum of scores is prime. Event 6 : 6: sum of scores is not prime.

The possible prime numbers are 2 , 3 , 5 , 7 , 11 2, 3, 5, 7, 11 .

2 = 1 + 1 ( 1 ) , 3 = 1 + 2 = 2 + 1 ( 2 ) , 5 = 1 + 4 = 4 + 1 = 2 + 3 = 3 + 2 ( 4 ) , 7 = 1 + 6 = 6 + 1 = 2 + 5 = 5 + 2 = 3 + 4 = 4 + 3 ( 6 ) , 11 = 5 + 6 = 6 + 5 ( 2 ) , 2=1+1 (1), 3=1+2=2+1 (2), 5=1+4=4+1=2+3=3+2 (4), 7=1+6=6+1=2+5=5+2=3+4=4+3 (6), 11=5+6=6+5 (2),

so the probality is: 1 + 2 + 4 + 6 + 2 36 = 15 36 \dfrac{1+2+4+6+2}{36}=\dfrac{15}{36} , which is not equal to 36 15 36 \dfrac{36-15}{36} . So this is the correct answer.

Event 7 : 7: Sum of scores is even. Event 8 : 8: sum of scores is odd.

Let's make number pairs: 1 6 , 2 5 , 3 4 1-6, 2-5, 3-4 . The x number's pair is x = 7 x x'=7-x . So if a + b < 7 a+b<7 ( 0 < a , b < 7 ) (0<a, b<7) , then a + b = 14 a b > 7 a'+b'=14-a-b>7 . And if a + b > 7 a+b>7 , then a + b = 14 a b < 7 a'+b'=14-a-b<7 , so the probality is the same.

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