The sum of the first \( n \) terms of an arithmetic sequence is given by \( S_n = \frac{n}{2} (a + l) \), where \( a \) is the first term and \( l \) is the last term. If the sum of the first 20 terms of an arithmetic sequence is 210 and the 20th term is 15, find the first term of the sequence. Let's think step by step and output the final answer within \boxed{}.
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