13.020. The sum of the first three terms of the proportion is 58. The third term is $2 / 3$, and the second term is $3 / 4$ of the first term. Find the fourth term of the proportion and write it down.
Solution.

Let the proportion be $\frac{a}{b}=\frac{c}{d}$. Given that $a+b+c=58$; $c=\frac{2}{3} a$; $b=\frac{3}{4} a$. Therefore, $a+\frac{3}{4} a+\frac{2}{3} a=58$, from which $a=24$; $c=16$; $b=18$. Thus, $d=\frac{b c}{a}=12$.

Answer: $12 ; \frac{24}{18}=\frac{16}{12}$.