Basic College Mathematics (10th Edition)

Ratio daughter to son is $\frac{4}{5}$. Other possibilities are $\frac{8}{10} ,\frac{12}{15}, \frac{16}{20}, \frac{20}{25}, \frac{24}{30}$ and $\frac{28}{35}$.
Ratio daughter to son is $\frac{4}{5}$. If the daughter is 8 years old, we can find out the ratio by $\frac{4 \cdot 2}{5 \cdot 2} = \frac{8}{10}$. So the son is 10 years old. If the daughter is 12 years old, we can find out the ratio by $\frac{4 \cdot 3}{5 \cdot 3} = \frac{12}{15}$. So the son is 15 years old. If the daughter is 16 years old, we can find out the ratio by $\frac{4 \cdot 4}{5 \cdot 4} = \frac{16}{20}$. So the son is 20 years old. If the daughter is 20 years old, we can find out the ratio by $\frac{4 \cdot 5}{5 \cdot 5} = \frac{20}{25}$. So the son is 25 years old. If the daughter is 24 years old, we can find out the ratio by $\frac{4 \cdot 6}{5 \cdot 6} = \frac{24}{30}$. So the son is 30 years old. If the daughter is 28 years old, we can find out the ratio by $\frac{4 \cdot 7}{5 \cdot 7} = \frac{28}{35}$. So the son is 35 years old.