Since the amount if Caffeine decreases by 12% an hour, that is the same as a multiplier of 0.88 per hour.
r = ln 0.88 = -0.117833371
10 = 120 e^-0.117833371t
10/120 = e^-0.117833371t
0.8333 = 10/120
ln0.8333 = -2.48490665 so
e^-2.48490665 = e^-0.117833371t and so
-2.48490665 = -0.117833371t
-2.48490665/-0.117833371 = t
21.08830995 hrs = t
thus 21 hrs is your best answer.
