How many different pairs of parallel edges are there on a rectangular solid?

18

Step-by-step explanation:

A rectangular prism has four parallel edges along its length, four parallel edges along its width, and four parallel edges along its height.

We want to know how many different pairs of parallel edges there are.  Starting with the length, the number of unique pairs is:

₄C₂ = 6

The same is true for the width and height.  So the total number of different pairs of parallel edges is:

3 × 6 = 18