A solid cube of 4 inches has been painted red, green and black on pair of opposite faces. It has been cut into one inch cubes. How many cubes have only one face painted?

As per the given all information in question , we can say that
In Layers 1 and 4 the four cubes each in the centre have only one face painted. In Layers 2 and 3, eight cubes each in the centre have one face painted .
Thus, total number of cubes = 8 + 16 = 24