if it takes ten minutes to travel 5 km, then it'll take 60 minutes to travel thirty, making your biking speed 30 kilometers per hour.

Apermeable rock is one that has tiny spaces connected in it which allow water to seep through those space and into the rock. as water continues to seep through the rock, it begins a slow and gradual process of rock erosion. since less permeable rock will have lesser spaces, less water will seep through it and thus it will erode more slowly than a rock which is relatively more permeable.