Quinten and his friends ate \( \frac{6}{10} \) of the peach pie.
To find the number of slices they ate from the cherry pie, we first need to determine how many slices \( \frac{6}{10} \) represents in the cherry pie.
Since the cherry pie is cut into 15 slices, we can calculate the fraction of the cherry pie they ate, which is also \( \frac{6}{10} \).
Now, find \( \frac{6}{10} \) of the 15 slices of the cherry pie:
\[ \frac{6}{10} \times 15 = \frac{6 \times 15}{10} = \frac{90}{10} = 9 \]
Thus, they ate \( 9 \) slices of the cherry pie.
The answer is \( 9 \).