



 
Hi Laura, I don't remember many mathematical formulas, especially when it comes to arithmetic series. I'll show you how I would approach this problem. I can see from the sequence 5, 9, 13, ... that each term is 4 more than the previous term.
Thus the n^{th} term must be n1 fours plus 5. That is t_{n} = 4(n  1) + 5 = 1 + 4n. Thus the n term series is
You can find the sum of this series by writing it forward and backwards and then adding down
Since there are n terms in the series
and thus
Solve for n.  


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