Answer:
It is not valid because she only saw 5 students and that's not a large enough sample for the whole school.
Step-by-step explanation:
While 4/5 IS 80%, the nurse assumed that 80% of ALL of the students in the school were coming down with colds. This is not true since only 4 students had colds. If she were specific and said that 4/5 students who visited her on Monday were 80% coming down with colds, she'd be right. But she isn't since she's assuming it's 80% of the whole school's population of students.
