Sum of a geometric series

[loxxii]

Find the sum (s) of a geometric series for which A1=48, An=3, and r=-1/2

Now, the formula is s = A1/1-r, so 48/1.5 = 32 is what I get.

However, the choices are
– 99, 33, 99, or 15

Why does the formula not use An and should I chose 33 since it is the closest? I am lost.