Question

A disinfectant spray bottle says it eliminates 90% of bacteria with each application. You apply the disinfectant once to a countertop in the shipping department and then apply the disinfectant again to the same countertop. If the bottle is correct, what percentage of the bacteria originally on the tabletop have been eliminated?

Answer 1

If you eliminate 90% of bacteria with the first application, then 10% of the bacteria will survive.

If you eliminate another 90% of bacteria remaining with the first application, then you will take 90% of 10 and subtract that to find the population remaining after the second application.

$.90 * .10 = .09$

$.10 - .09 = .01$

Only 1% of the bacteria is left. Therefore, 99% of the bacteria has been eliminated.