It takes \(\dfrac{35}{30}=\dfrac{7}{6}\left(h\right)=70'\) to drive all this distance at 30mi/h

It takes \(\dfrac{25}{30}=\dfrac{5}{6}\left(h\right)=50'\)to drive all this distance at 25mi/h

So drive at 30mi/h is takes fewer than 25mi/h is: 70' - 50' = 20'

Answer: 20 minutes