7. The first three terms of an arithmetic series are \(( 12 - p ) , 2 p\) and \(( 4 p - 5 )\) respectively, where \(p\) is a constant.
- Find the value of \(p\).
- Show that the sixth term of the series is 50 .
- Find the sum of the first 15 terms of the series.
- Find how many terms of the series have a value of less than 400.