Bacteria reproduce by binary fission, a process by which one parent cell divides to form 2 progeny cells. Because one cell produces 2 progeny cells, bacteria are said to undergo exponential ( logarithmic) growth.
As in the case of staphylococcus aureus, we start with 100 cells , and after 90 minutes number becomes 800 that means , firstly the cells doubled (100 x 2 = 200 ) after 30 minutes. Again after 30 minutes they doubled ( 200 x 2 =400 ) and again after 30 minutes they doubled (400 x 2 = 800). So in 90 minutes 100 cells became 800 cells.
Hence the generation time for S.aureus is 30 minutes.
