A missing helicopter is equally likely to have crashed anywhere in the rectangular region shown in Fig. 14.2. What is the probability that it crashed inside the lake? 

Use area ratio because every point in the region is equally likely. The full rectangle has dimensions $9\text{ km} \times 4.5\text{ km}$, so its area is $40.5\text{ km}^2$. The lake is a rectangle of dimensions $3\text{ km} \times 2.5\text{ km}$, so its area is $7.5\text{ km}^2$. Therefore, the probability is $\frac{7.5}{40.5}=\frac{75}{405}=\frac{5}{27}$.