Chapter 1, Equations and Graphs - Section 1.10 - Modeling Variation - 1.10 Exercises - Page 164: 30

$M=32 \frac{abc}{d}$

Since $M$ is jointly/directly proportional to $a$, $b$, and $c$, and inversely proportional to $d$, we have: $M=k \frac{abc}{d}$ We are given: $M=128$ and $a=d$ and $b=c=2$, so we have: $128=k \frac{a*2*2}{a}=4k$ $k=128/4=32$ So the equation is: $M=32 \frac{abc}{d}$