how is 50S and 30 S = 70S possible? 50+30=80

A ribosome is a complex organelle present inside all cells. It is a site for protein synthesis. Each ribosome is divided into two units – one small unit of 30S and bigger unit of 50S. Here ‘S’ is Svedberg unit, which measures the rate of sedimentation in centrifugation. It does not measure size. Thus, here the units of ribosome cannot be added or summed up to get 80S  (50S + 30S). 

In prokaryotes the subunits 30S and 50S join to get 70S ribosome. Eukaryotes have 80S ribosome with subunits 40S and 60S.