4 An arithmetic series has first term \(a\) and common difference \(d\).
The sum of the first 31 terms of the series is 310 .
- Show that \(a + 15 d = 10\).
- Given also that the 21st term is twice the 16th term, find the value of \(d\).
- The \(n\)th term of the series is \(u _ { n }\). Given that \(\sum _ { n = 1 } ^ { k } u _ { n } = 0\), find the value of \(k\).