f(x) = 50 * 1.5^x (not 1.5x): # of bacteria present.
Average Rate of Change from 0 days to 4 days is
f(4) - f(0) 50(1.5^4 - 1.5^0) bacteria
------------- = -------------------------- ------------------
4-0 4 day
50(5.0625 - 1)
= ----------------------- = 51 bacteria per day = average rate of change
4
