Final answer:
Dust does not have a third type of charge; it is usually neutral. Both positively charged glass rods and negatively charged silk attract dust through electrostatic induction, which causes a charge separation in the dust particles.
Step-by-step explanation:
When a glass rod is rubbed with silk, the glass rod becomes positively charged and the silk becomes negatively charged. This is because electrons are transferred from the glass to the silk, leaving an excess of protons (which carry a positive charge) on the glass and an excess of electrons (which carry a negative charge) on the silk. However, the dust that both the rod and the silk attract does not have a third type of charge.
Dust is typically neutral, but it can become charged through friction or by interaction with charged objects. When the positively charged glass rod or the negatively charged silk come close to the dust, they may induce a charge separation within the dust particles. This induced charge causes one side of the dust particle to become more positive or more negative, allowing it to be attracted to the charged object. The phenomenon is known as electrostatic induction.