A geometric sequence g starts 80, 40, ...

1.  The common difference is (1/2)

Recursive

a_n  =  (1/2)a_n-1, a1  = 80

2.  Here: https://www.desmos.com/calculator/dzbwv2cbw4

3. Recursive...tedious....find the first 99 terms....the 100th term = (1/2) * 99th term