Question 14
Sign of a product and sign of a quotient: $a$, $b$, $c$ and $d$ are four non-zero numbers such that the quotients $\frac{a}{5}$, $\frac{-b}{7 a}$, $\frac{11}{a b c}$, $\frac{-18}{a b c d}$ are positive. Determine the signs of $a$, $b$, $c$ and $d$.