The answer that is shown in Fig. 1, where the curved wall shuts out the cottages from access to the lake is a possibility.
But in seeking the direction for the "shortest possible" wall most readers to-day, remembering that the shortest distance between two points is a straight line, will adopt the method shown in Fig. 2. 
This is certainly an improvement, yet the correct answer is really that indicated in Fig. 3. 
A measurement of the lines will show that there is a considerable saving of length in this wall.