其它
Ella says:" I can run $18\text{km}$ every hour."  Vivian says: " Haha, I am the faster runner because I can run $100\text{m}$ every $21\text{s}$."  Vivian\textquotesingle s statement is~\uline{~~~~~~~~~~}~because it only takes Ella~\uline{~~~~~~~~~~}~$\text{s}$ to run $100\text{m}$. 
Overseas Competition->Knowledge Point->Word Problem Modules->Solving Problems Involving Fractions and Percentages->Finding the Rate
$\frac{18km}{1h}=\frac{18000m}{3600s}=5m/s$  $100\div 5=20s$ 