# Fairy Straightforward Proportionality Problem

• Oct 17th 2009, 03:27 PM
Togechu64
Fairy Straightforward Proportionality Problem
Hey Guys,

This one doesn't involve any real math all too much, but I'm still getting confused on it, help? Thanks!

The rate R that a certain disease spreads is proportional to the number of infected individuals and is also proportional to the number of uninfected individuals. The total population is P and the number of infected individuals is D. Express the rate that the disease spreads in terms of this information. Use C as your proportionality constant.
R(P,D)=_______________

(EDIT: I can't believe I misspelled "fairly" in the title, sorry!)
• Oct 17th 2009, 09:32 PM
Hello Togechu64
Quote:

Originally Posted by Togechu64
Hey Guys,

This one doesn't involve any real math all too much, but I'm still getting confused on it, help? Thanks!

The rate R that a certain disease spreads is proportional to the number of infected individuals and is also proportional to the number of uninfected individuals. The total population is P and the number of infected individuals is D. Express the rate that the disease spreads in terms of this information. Use C as your proportionality constant.
R(P,D)=_______________

(EDIT: I can't believe I misspelled "fairly" in the title, sorry!)

If there are $D$ individuals infected, and $P$ in total, then there are $(P-D)$ uninfected. So $R$ is proportional to $P$ and to $(P-D)$; i.e. $R = CD(P-D)$.