Chapter 6 - Section 6.2 - Proportions - Exercise Set - Page 427: 29

$\frac{10}{15} = \frac{4}{6}$ True

x is to y is the same as $\frac{x}{y}$ To determine if a proportion is true, we have to find if the cross products are equal. The cross products of $\frac{a}{b}=\frac{c}{d}$ are $a \cdot d$ and $b \cdot c$. Ten is to fifteen as four is to six is the same as $\frac{10}{15} = \frac{4}{6}$ This proportion is true because $10*6=4*15=60$