4. \( \frac{18}{25}=\frac{126}{} \)

To find the missing number in the proportion \( \frac{18}{25}=\frac{126}{x} \), you can use cross-multiplication. This means you multiply 18 by \( x \) and 25 by 126: \( 18x = 25 \times 126 \). Calculating \( 25 \times 126 = 3150 \), we get: \( 18x = 3150 \). Now, divide both sides by 18: \( x = \frac{3150}{18} = 175 \). So, the answer is \( x = 175 \). And just like that, you’ve solved a proportion! Think of it as balancing a scale—what you do to one side, you must do to the other.