We are told that T(0)=25, so use this to find C:
Now, use the condition T(20)=15 to find k, then set the equation equal to 5 and solve for t.
guys, i don't know how to solve this problem, i know i have to use dT/dt = -kT.
A pitcher of butter milk initially at 25C is to be cooled by setting it on the front porch, where the temperature is 0C. Suppose that the temperature of the buttermilk has dropped to 15C after 20 min. When it will be at 5C?