In an arithmetic sequence, the nth term $a_n$ is given by the formula $a_n=a_1+(n-1)d,$ where $a_1$ is the first term and $d$ is th given by $a_n=a_1*r^{n-1}.$ Here $r$ is the common ratio. Use these formulas to determine the indicated term in the given sequ

The $18$ th term of $16,41,66,91,$dots

The $18$th term of the sequence is $.($Simplify your answer.$)$