The temperature of coffee 10 minutes after it was poured (120 F) will help in solving the constant “a”.
120 = 120(2 ^ 10a) + 60
2 ^ -1 = 2 ^ 10a
a = -1/10
The temperature of coffee 30 minutes after it was poured is:
F = 120 ( 2 ^ -30/10) + 60
F = 120 * 1/ 8 + 60
F = 15 + 60
F = 75