Final answer:
Micaela had 26 posters on Monday.
Step-by-step explanation:
To find the answer, we need to work backwards from the information provided. On Tuesday, Micaela bought 6 posters. On Wednesday, half of all the posters she had were destroyed, which left her with 6/2 = 3 posters remaining. Finally, on Thursday, there were only 23 posters left. Therefore, she must have had 23 + 3 = 26 posters on Monday.