Chapter 1, Equations and Graphs - Section 1.10 - Modeling Variation - 1.10 Exercises - Page 164: 23

$A= \frac{18x}{t}$

Since $A$ is directly proportionaI to $x$ and inversely proportional to $t$, we have: $A=\frac{kx}{t}$ We are given: $A=42$ and $x=7$ and $t=3$, so we have: $42= \frac{k*7}{3}$ $k=42*3/7=18$ So the equation is: $A= \frac{18x}{t}$