3 A sequence of terms \(u _ { 1 } , u _ { 2 } , u _ { 3 } , \ldots\) is defined by
$$u _ { n } = 24 - \frac { 2 } { 3 } n$$
- Write down the exact values of \(u _ { 1 } , u _ { 2 }\) and \(u _ { 3 }\).
- Find the value of \(k\) such that \(u _ { k } = 0\).
- Find \(\sum _ { n = 1 } ^ { 20 } u _ { n }\).