3 A geometric series begins
$$20 + 16 + 12.8 + 10.24 + \ldots$$
- Find the common ratio of the series.
- Find the sum to infinity of the series.
- Find the sum of the first 20 terms of the series, giving your answer to three decimal places.
- Prove that the \(n\)th term of the series is \(25 \times 0.8 ^ { n }\).