Chapter 4 - Proportions, Percents, and Solving Inequalities - 4.1 - Ratios and Proportions - Problem Set 4.1 - Page 149: 49

$n=120$

Using cross-multiplication, the value of the variable that satisfies the given equation, $ \dfrac{300-n}{n}=\dfrac{3}{2} ,$ is \begin{array}{l}\require{cancel} 2(300-n)=3n \\\\ 600-2n=3n \\\\ -2n-3n=-600 \\\\ -5n=-600 \\\\ n=\dfrac{-600}{-5} \\\\ n=120 .\end{array}