b1=8 q=1.2 b2=8*1.2=9.6 b3=9.6*1.2=11.52 b4=11.52*1.2=13.824 b5=13.824*1.2=16.5888 S5=b5*q-b1/q-1 S5=16.5888*1.2-8/1.2-1=59.5328