Three candidates ran for the election of a class representative at a school: João, Rosa, and Marcos. João received $2 / 7$ of the votes and Rosa $2 / 5$ of the votes. Who won the election?
João received $2 / 7$ of the total votes, Rosa received $2 / 5$ of the total votes, and Marcos received $1-(2 / 7+2 / 5)=1-24 / 35=11 / 35$ of the total votes. The winner was the one who obtained the largest fraction of the votes. To compare these fractions, we equalize their denominators, obtaining $2 / 7=10 / 35$ and $2 / 5=14 / 35$. Thus, we have

$$
\underbrace{\frac{2}{7}}_{\text {João }}<\underbrace{\frac{11}{35}}_{\text {Marcos }}<\underbrace{\frac{2}{5}}_{\text {Rosa }}
$$

and, therefore, Rosa won the election. (It is interesting to note that the answer does not depend on the number of students in the class.)