This GRE Math practice problem tests your understanding of averages(arithmetic mean) in statistics.

The average (arithmetic mean) of a list of thirteen numbers is $17$. A new number is added to the list, and the average (arithmetic mean) of this new set of numbers is $18$. What number has been added?

- $\quad 13$
- $\quad 27$
- $\quad 31$
- $\quad 39$
- $\quad \textrm{It cannot be determined based on the information above}$

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