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The sequence $a_{1}, a_{2}, a_{3}, a_{4}, a_{5}$ is such that $a_{n} = a_{n-1} + 5$ for $2 \leq n \leq 5$. If $a_{5} = 31$, what is the value of $a_{1}$?
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an = a(n-1) + 5

a(n-1) = an - 5

a4 = a5 - 5 = 31 - 5 = 26

a3 = a4 - 5 = 26 - 5 = 21

a2 = a3 - 5 = 21 - 5 = 16

a1 = a2 - 5 = 16 - 5 = 11
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