8.5% means that, every hour, there are 91.5% remaining from the previous hour.........so the function is :
y = 650(.915)^x where y is the number of bacteria present after x hours
The domain is [0, infinity (theoretically) )
The range is (0, 650]
Here's the graph : https://www.desmos.com/calculator/pwxvqmil5o
Half of the bacteria remains when y = 325........this occurs at about 7.8 hours from the start
