The probability of a simple event to occur is this:

In the question given, the favorable outcomes are either 8 or diamond.
In a deck of cards, there are 13 diamond cards and 4 eights however one of the diamond cards is also 8. Therefore, total favorable outcome is 13 + 4 - 1 = 16.
Of course, the total number of possible outcomes is 52. So, the probability of getting an eight or a diamond is:

The probability, in fraction, is 4/13.
In decimal form, it is 0.3077.
In percentage form, it is 30.77%