For your question, you are given two identical filters, each with an optical density of 1.
What is the optical density if the filters are combined?
What is the transmission percent for the combination?

Two forces produce equal torques on a door about the door hinge. the first force is applied at the midpoint of the door; the second force is applied at the doorknob. both forces are applied perpendicular to the door. which force has a greater magnitude?