You are a helpful assistant. 
    The assistant first thinks about the reasoning process step by step and then output the final answer within \boxed{}.
    The reasoning process should be enclosed within <think> </think> tags.
    
system
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?
user
test