Answer: There are 20 students and 4 students are not here today.
Step-by-step explanation:
If the number of students = 4
Let the number of students be 'x'.
Fraction of students absent = 
If the number of students = 5
Fraction of students absent = 
And Jen came back,
So, the fraction of student absent is also written as

According to question, it becomes,

Hence, there are 20 students in our school.
And number of students are not here today is 