The sum of the first 5 terms of an arithmetic series is $70$.  The sum of the first 10 terms of this  arithmetic series is $210$.  What is the first term of the series?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
The sum of the first 5 terms of an arithmetic series is $70$.  The sum of the first 10 terms of this  arithmetic series is $210$.  What is the first term of the series?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\frac{42}{5}
The sum of the first 5 terms of an arithmetic series is $70$.  The sum of the first 10 terms of this  arithmetic series is $210$.  What is the first term of the series?
train