CEMC Euclid, 2009

Algebra Difficulty 2.0 Junior Prove it Canada

In an arithmetic sequence, the first term is $1$ and the last term is $19$ . The sum of all the terms in the sequence is $70$ . How many terms does the sequence have? (An arithmetic sequence is a sequence in which each term after the first is obtained from the previous term by adding a constant. For example, $3, 5, 7, 9$ is an arithmetic sequence with four terms.)

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