Let's denote the time Wendy spent prepping her pie as \( W \) minutes. According to the problem, Wendy baked her pie for 60 minutes, so the total time it took for Wendy to finish her pie is:
\[ W + 60 \]
Finn spent twice as much time prepping as Wendy, so his prep time is \( 2W \) minutes. He then baked his pie for 35 minutes, making his total time:
\[ 2W + 35 \]
Since both pies finished at the same time, we can set their total times equal to each other:
\[ W + 60 = 2W + 35 \]
To solve for \( W \), we first simplify the equation:
\[ 60 - 35 = 2W - W \] \[ 25 = W \]
Thus, Wendy spent 25 minutes prepping her pie.