There are 3 working weeks and each work week she earns $1250, so the total earning is:
total = 1250 * 3 = $3750
For each day of late, she is fined $100, and she must make at least $3,000. So she can only be late for 7 days so that her earning would be:
earning = 3750 – 7*100 = $3050
Max number of days late = 7days