Jello: T(t) = 36 + (85 - 36)e^-kt
where t = time taken for the cooling
T(t) = temperature of the jello at time t
k = decay constant

a) Temperature after 20 minutes = 60º F.
T(20) = 60 = 36 + (85 - 36)e^-k•20
60 = 36 + 49e^-20k
24 = 49e^-20k
24/49 = e^-20k
ln(24/49) = ln e^-20k
-0.7137664678 = -20k
0.0356883234 = k

b) T(t) = 40 = 36 + 49e^{-0.0356883234t}
4 = 49e^{-0.0356883234t}
4/49 = e^{-0.0356883234t}
ln(4/49) = ln e^{-0.0356883234t}
-2.505525937 =-0.0356883234t
70.20576195 ≈ t ≈ 70 minutes