$a, b, c$ and $d$ are four non-zero numbers such that the quotients

$$
\frac{a}{5}, \quad \frac{-b}{7 a}, \quad \frac{11}{a b c} \quad \text { and } \quad \frac{-18}{\mathrm{abcd}}
$$

are all positive. Determine the signs of $a, b, c$ and $d$.
- Since $\frac{a}{5}>0$ and $5>0$, we obtain $a>0$.
- Since $a>0$, we have $7a>0$. Since $\frac{-b}{7a}>0$, it follows that $-b>0$, therefore, $b<0$.
- Since $a>0$ and $b<0$, and $c>0$ and $11>0$, we obtain $abc>0$. Since $b<0$ and $-18<0$, it follows that $d<0$.