To find out how many slices of cherry pie they ate, we first need to determine the fraction of the peach pie that was eaten.
Quinten and his friends cut a peach pie into 10 equal slices and they ate 6 slices of it.
The fraction of the peach pie eaten is: \[ \text{Fraction of peach pie eaten} = \frac{6 \text{ slices}}{10 \text{ slices}} = \frac{3}{5} \]
Now, we will apply the same fraction to the cherry pie, which has 15 equal slices.
To find out how many slices of cherry pie they ate, we take: \[ \text{Slices of cherry pie eaten} = \frac{3}{5} \times 15 \]
Calculating this gives: \[ \text{Slices of cherry pie eaten} = 3 \times 3 = 9 \]
Therefore, the number of slices of cherry pie they ate is 9.