I learned optimization (distance) but I missed the lesson on optimizing time so I'm having some trouble on this question:

A sailor in a boat 8 km offshore wants to reach a point on shore that is 10 km from the point that is directly opposite his current position in the shortest possible time. Find the landing point and traveling time if:

a) She rows at 4 km/h and runs at 6 km/h

b) She rows at 4 km/h and walks at 5 km/h (watch the Domain on this one)

I'm not sure how optimizing time differs from distance, so if someone could give me a quick explanation on part a I'll try part b. I know $\displaystyle \displaystyle time=\frac{distance}{velocity}$, but I'm not sure how to set up a relevant equation.

Thanks for the help!