1. The problem asks to find the fourth proportional of the numbers 15, 18, and 25. 2. The fourth proportional $x$ to three numbers $a$, $b$, and $c$ is defined by the proportion $$\frac{a}{b} = \frac{c}{x}$$ 3. Rearranging the formula to solve for $x$, we get $$x = \frac{b \times c}{a}$$ 4. Substitute the given values $a=15$, $b=18$, and $c=25$ into the formula: $$x = \frac{18 \times 25}{15}$$ 5. Calculate the numerator: $18 \times 25 = 450$ 6. Divide by the denominator: $\frac{450}{15} = 30$ 7. Therefore, the fourth proportional is $30$.