Awoman at a point a on the shore of a circular lake with radius r=4 wants to arrive at the point c diametrically opposite a on the other side of the lake in the shortest possible time. she can walk at the rate of 10mph and row a boat at 5mph. what is the shortest amount of time it would take her to reach point c?
i'm not sure where to : (

Attached is a drawing to set up the problem, hopefully that helps. 
The red lines are tracing the rowing and walking to get to other side.
dR = distance rowed
dw = distance walked
x = the angle in which to start rowing initially 

dR can be found by using Law of cosines

dw is arc length, which is just radius*arc


next we need t set up a function for Time in terms of x.
This is so we can minimize the time it takes to get to other side.
Time = distance/rate


Finally, take derivative and set equal to 0
Once you solve for x, plug it back into Time function to obtain final answer.