$$\frac{3}{7}$$ of the passengers on the bus were adults and the rest were children. There were $$24$$ children. How many adults were there? 
Overseas Competition->Knowledge Point->Word Problem Modules->Solving Problems Involving Fractions and Percentages->Finding the Base
$$1- \frac{3}{7}= \frac{4}{7}$$  $24\div \frac47=42$  $42-24=18$. 