6 A sequence \(u _ { 1 } , u _ { 2 } , u _ { 3 } , \ldots\) is defined by \(u _ { n } = 85 - 5 n\) for \(n \geqslant 1\).
- Write down the values of \(u _ { 1 } , u _ { 2 }\) and \(u _ { 3 }\).
- Find \(\sum _ { n = 1 } ^ { 20 } u _ { n }\).
- Given that \(u _ { 1 } , u _ { 5 }\) and \(u _ { p }\) are, respectively, the first, second and third terms of a geometric progression, find the value of \(p\).
- Find the sum to infinity of the geometric progression in part (iii).