0-255 Levels for RGB Composit

How does Adobe Photoshop arrive at the RGB composit 0-255 level value for a pixel, given the individual levels (0-255) for each of the colors Red, Green, Blue for the same pixel? Is it a simple average?

For example, invoking Image|Adjustments|Levels will bring up a histogram of these levels. How is the RGB composit level calculated for a pixel given the individual R, G, B levels for that pixel?

That depends on the blending mode, pixel transparency and opacity settings.

And the exact math is pretty long and complex.

Is it a linear combination, or does it involve non-linear expressions?

That depends on the blending mode.