We apply the concept of calculus here. You can determine the minimum or maximum in here if you take the derivative of a function and equate it to zero. The solution is as follows:
n = 20t² - 20t +120
dn/dt = 0 = 40t - 20 + 0
40t - 20 = 0
40t = 20
t = 20/40
t = 1/2 or 0.5
Therefore, the number of bacteria would be minimal at 0.5°C.
