When the opacity of the top layer is lowered, how does Photoshop determine the RGB pixel values?

As an example, the top layer's RGB values for a given pixel are:
R=11
G=208
B=206
Under the same layer, the same pixel can be found as follows (by turning off the visibility of the top layer):
R= 195
G=57
B=63
When the top layer's visibility is turned back on and it's opacity is changed to 80%, the new values for the same pixel are now:
R=71
G=184
B=183
What is Photoshop's method for calculating the new values?

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