Final answer:
A 20:B extinguisher can extinguish 10 times as much fire as a 2:B extinguisher because the ratings indicate the relative capacity to fight a fire, with each unit representing a multiplier of the agent needed for a baseline level of fire.
Step-by-step explanation:
The question involves understanding the ratings on fire extinguishers, which are indicators of the capacity of an extinguisher to fight a fire. The rating 20:B means the extinguisher can handle a fire that would typically require 20 times as much agent as a fire with a 1:B rating. In comparison, a 2:B extinguisher can handle a fire that would typically require twice as much agent as a fire with a 1:B rating. Therefore, a 20:B extinguisher would be able to extinguish 10 times as much fire as a 2:B extinguisher, because if you divide 20 by 2, you get 10.