If Quinten and his friends ate \( \frac{6}{10} \) of the peach pie, we first convert that fraction into slices. The peach pie has 10 slices:
\[ \frac{6}{10} \times 10 = 6 \text{ slices} \]
Now, they ate the same fraction of the cherry pie, which is also \( \frac{6}{10} \). The cherry pie has 15 slices:
\[ \frac{6}{10} \times 15 = \frac{6 \times 15}{10} = \frac{90}{10} = 9 \text{ slices} \]
So, they ate 9 slices of the cherry pie.
The correct answer is 9.