# Probability! Two tetrahedral dice are thrown

thornluke

## Homework Statement

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?

## The Attempt at a Solution

a) P(even) = 1/2
b) P(prime) = 9/16

c) c for confused

Cheers.

Homework Helper
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?

thornluke
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?

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