Solution:

The answer is $\mathbf{( E )}$. One way to realize this consists in showing two different subdivisions of the course into three parts for which the conditions of the problem are satisfied. For example: the athlete could have swum for $\frac{7}{27}$ of an hour, run for one hour and cycled for $\frac{11}{27}$ $\mathrm{of}$ an hour. In that case he would have competed for $\left(\frac{7}{27}+1+\frac{11}{27}\right)$ hours $=\left(1+\frac{2}{3}\right)$ hours $=100$ minutes and would have covered a distance equal to
$$\left(3 \frac{7}{27}+17+30 \frac{11}{27}\right) \mathrm{Km}=30 \mathrm{Km}$$
Or he could have swum for half an hour, run for half an hour and cycled for 40 minutes (which are $\frac{2}{3}$ of an hour). In that case too the time of the race would have been one hour and forty minutes and the course length
$$3 \frac{1}{2}+17 \frac{1}{2}+30 \frac{2}{3}=30 \mathrm{Km}$$