Solution

The idea is to determine the value of watermelons in terms of jackfruits. Since there are $18+30=$ 48 watermelons and 24 jackfruits, we have the proportion

$$
\begin{array}{ccc}
48 \text { watermelons } & - & 24 \text { jackfruits } \\
1 \text { watermelon } & - & x \text { jackfruits }
\end{array}
$$

which gives $x=1 / 2$. That is, one watermelon is equivalent to half a jackfruit. Since Renato had 30 watermelons, he should receive 15 jackfruits. Since Leandro had 18 watermelons, he should receive 9 jackfruits.
Renato has thirty watermelons, Leandro has eighteen watermelons, and Marcelo has twenty-four jackfruits. Unlike Leandro and Renato, Marcelo does not like jackfruit. On the other hand, all three like watermelons. The three then make an agreement: Marcelo gives his twenty-four jackfruits to Leandro and Renato, and the watermelons of Leandro and Renato are divided equally among the three, that is, sixteen for each. What is the fair division of jackfruits between Renato and Leandro?