The sum of the first \( n \) terms of an arithmetic series is given by the formula \( S_n = \frac{n}{2} (2a + (n-1)d) \), where \( a \) is the first term and \( d \) is the common difference. If the sum of the first 10 terms of an arithmetic series is 145, and the sum of the first 20 terms is 380, what is the 30th term of the series? Let's think step by step and output the final answer within \boxed{}.
user