Chapter 2 Linear Equations and Functions - 2.5 Model Direct Variations - 2.5 Exercises - Problem Solving - Page 111: 44c

Only part a is direct variation

The equation in part b is not direct variation, for it is: $$d_2=6000-75t$$ This is not in the form $y=ax$. However, the equation in part a ($d_1=75t)$ is in the correct form, so it is direct variation.