Tobias has a job walting tables where he earns a certain amount every month and an additional amount per table, according to the
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The 1st row has 15 chairs.
The 2nd row has 15 + 3 = 18 chairs.
The 3rd row has 18 + 3 = 15 + 2•3 = 21 chairs.
The 4th row has 21 + 3 = 15 + 3•3 = 24 chairs.
The pattern continues, so that the n-th row has 15 + (n - 1)•3 = 3n + 12 chairs.
If there are 15 rows, then the total number of chairs is
where we use the formulas