Chapter 9 - Section 9.4 - Variation and Problem Solving - Exercise Set - Page 662: 18

The constant of variation is $4$. The inverse variation equation is $y=\frac{4}{x}$.

$y=\frac{k}{x}$ $y=\frac{1}{10}; x=40$ $\frac{1}{10}=\frac{k}{(40)}$ $\frac{1}{10}\times40=k$ $4=k$ The constant of variation is $4$. The inverse variation equation is $y=\frac{4}{x}$.