Calculus, AP?
caluculus, fouth, ration, are not words normally used by Calculus students. Choosing words, and checking the spelling, reflects on you personally. If you desire success, work on these types of details. You can do better than this.
Given that a3 is 16, and a4 is 8, the ratio must be r=.5
a3=a0*r^(3-1)
16=a0*(1/2)^(3-1)or a0=16*4
check: 16*4, 16*2, 16*1, 16*1/2, ...
sum:
Sum= ao/(1-r)=16*4/.5=16*8
check my thinking.