Answer: 90.
Step-by-step explanation: According to the question above, 54 Thin Mints cookies were 60% of the total amount of cookies she sold in one afternoon and the remaining 40% is still missing. 54 / 3 is 18, which is part of our answer, because if 54 is 60%, then 18 is 20%. 18 x 2 is 36, and 36 is 40%. Now that we've found the remaining 40%, the rest is adding 54 and 36, which gives you an answer of 90.