13. The triathlon consists of swimming, cycling and running. The cycling accounts for three-quarters of the total distance, the running for one-fifth and the swimming for $2 \mathrm{~km}$. What is the total distance of this triathlon?
A $10 \mathrm{~km}$
B $20 \mathrm{~km}$
C $38 \mathrm{~km}$
D $40 \mathrm{~km}$
E $60 \mathrm{~km}$
13. D The cycling and running account for $\frac{3}{4}+\frac{1}{5}=\frac{15}{20}+\frac{4}{20}=\frac{19}{20}$ of the distance. So the swimming, which is $2 \mathrm{~km}$, is the remaining $\frac{1}{20}$ of the distance. Hence the distance is $20 \times 2=40 \mathrm{~km}$.