Suppose that the 8 identical faces of a regular octahedron, like the one shown below, are numbered from 11 through 18, with 1 number per face, and each face is equally likely to land down when the octahedron is tossed. What is the probability that, on 1 toss of this octahedron, the number on the face landing down is a prime number or an even number?

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The numbers that are even or prime from 11-18 are:

$$ 11, 12, 13, 14, 16, 17, 18 $$

This is 7 out of the 8 possible outcomes.