Why do 60S & 40S ribosomal subunits make an 80S ribosome (not 100S)?

Question

Why do 60S & 40S subunits make an 80S (not 100S) ribosome and, similarly, 50S & 30S make 70S? 60+40 is not equal to 80, nor is 50+30 equal to 70, so why are the subunits of the 80S and 70S ribosomes named so weirdly? I couldn't come up with any other mathematical explanation.

Answer 1

From the Internet:

Ribosomal particles are denoted according to their sedimentation coefficients in Svedberg units.

There is no reason to add sedimentation coefficients.