Since there are 4 sick students with flu for each sick student with other illness, then, there are 4 students with flu for each 5 sick students.
Multiply the ratio 4:5, which is the proportion of students with flu to sick students, by the total amount of sick students, to find how many students had the flu:
[tex]\frac{4}{5}\times60=48[/tex]Therefore, 48 students had the flu.
