Using a sixteen-sided number cube, what is the probability that you will roll an even number or an odd prime number? round to three decimals

When there is a probability this OR that, the individual probabilities are added.

probability of rolling an even number is 1/2

prime numbers between 1-16 are {2, 3, 5, 7, 11, 13} of these five are odd.

probability of rolling an odd prime number
5/16

probability of even OR odd prime
1/2 + 5/16
8/16 + 5/16 = 13/16

13/16 = 0.825