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?
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Event 3 : Sum of scores is even. Event 4 : sum of scores is odd.
There are three odd and three even numbers between 1 , 2 , 3 , 4 , 5 , 6 . Since even + even = even = odd + odd , from the 6 ∗ 6 = 3 6 ways, 3 ∗ 3 + 3 ∗ 3 = 1 8 ways are even, which is equal to 3 6 − 1 8 = 1 8 . So Event 3 and 4 have the same probality.
Event 1 : Sum of scores is 2 : . vent 2 : sum of scores is 1 2 : .
The 2 can only produced by 1 + 1 , and 1 2 only by 6 + 6 , so they have the same probality.
Event 5 : Sum of scores is prime. Event 6 : sum of scores is not prime.
The possible prime numbers are 2 , 3 , 5 , 7 , 1 1 .
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 ) , 1 1 = 5 + 6 = 6 + 5 ( 2 ) ,
so the probality is: 3 6 1 + 2 + 4 + 6 + 2 = 3 6 1 5 , which is not equal to 3 6 3 6 − 1 5 . So this is the correct answer.
Event 7 : Sum of scores is even. Event 8 : sum of scores is odd.
Let's make number pairs: 1 − 6 , 2 − 5 , 3 − 4 . The x number's pair is x ′ = 7 − x . So if a + b < 7 ( 0 < a , b < 7 ) , then a ′ + b ′ = 1 4 − a − b > 7 . And if a + b > 7 , then a ′ + b ′ = 1 4 − a − b < 7 , so the probality is the same.