If $x$, $y$ and $z$ represent the money held by Dan, Diane and Donna, then $y=15-z$ and $x=20-y=20-(15-z)=5+z$. We can let $z$ take on any value from $0$ to $15$ without any of the three amounts being negative, since presumably middle-schoolers are too young to assume debt.
Then the total capital held by the three is $x+y+z=(5+z)+(15-z)+z=20+z$. So their combined holdings can range anywhere from \$20 (Donna is broke) to \$35 (Donna is flush).
We will have more to say about this situation in Section TSS, and specifically Theorem CMVEI.