Answer:
The correct answer is B) 18%
Step-by-step explanation:
Note that the vaccine is already and is 80% effective. This means that if the entire population was administered with the vaccine, only 8/10 would gain immunity leaving 2/10 or 0.2 exposed.
If the new vaccine, therefore, is supposed to be 90% effective, it is only pragmatic to administer it against those who do not have immunity which is 0.2 of the existing population.
So to get the percentage that will develop immunity, we have:
0.2 x 0.9 = 0.18 or 18%
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