Homework Help: Probability! Two tetrahedral dice are thrown

1. Sep 2, 2012

thornluke

1. The problem statement, all variables and given/known data
Two tetrahedral dice (four-sided dice) are thrown. What is the probability that the sum of the scores is:
a) even
b) prime
c) even or prime?

2. Relevant equations

3. The attempt at a solution
a) P(even) = 1/2
b) P(prime) = 9/16

c) c for confused

Cheers.

2. Sep 2, 2012

HallsofIvy

Have you written out the set of "outcomes"? A tetrahedron has four sides so it has the numbers 1, 2, 3, and 4. The possible outcomes when two are thrown are (1, 1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3), (2, 4), (3, 1), (3, 2), (3, 3), (3, 4), (4, 1), (4, 2), (4, 3), and (4, 4). There are, of course, $4^2= 16$ outcomes and each is equally likely. How many have sums that are either even or prime?

3. Sep 2, 2012

thornluke

Ah... 2 is a prime number too.. I see now.
My mind has gone mad from all these probability questions..

4. Sep 3, 2012

HallsofIvy

It wasn't until now that I worked out the answer to (c) myself and I was very surprised at the answer!

5. Sep 3, 2012

Staff: Mentor

There is an easy way to find this probability:
How many odd, non-prime numbers are possible as sum?
All numbers which are not [odd and not prime] are even or prime.