1) 8+0.5*6=11

2) A1 = 11.8

    A2 = 12.4

d=a2-a1=12.4-11.8=0.6

a3 = 12.4+0.6=13

(n-1)*0.6=20.8-11.8

(n-1)=9/0.6

n-1=15

n=16

3) 28