9. On John's 10th birthday he received the first of an annual birthday gift of money from his uncle. This first gift was \(\pounds 60\) and on each subsequent birthday the gift was \(\pounds 15\) more than the year before. The amounts of these gifts form an arithmetic sequence.
- Show that, immediately after his 12th birthday, the total of these gifts was \(\pounds 225\)
- Find the amount that John received from his uncle as a birthday gift on his 18th birthday.
- Find the total of these birthday gifts that John had received from his uncle up to and including his 21st birthday.
When John had received \(n\) of these birthday gifts, the total money that he had received from these gifts was \(\pounds 3375\)
- Show that \(n ^ { 2 } + 7 n = 25 \times 18\)
- Find the value of \(n\), when he had received \(\pounds 3375\) in total, and so determine John's age at this time.