Last time, one out of every 4 students in our class was absent. this time, after jen came back only one out of every 5 students is absent. how many students are there in our class? how many students are not here today?

Let x be the number of students in the class the
 the number of students in the class when 1 in 4 are absent = x - 0.25x  = 0.75x
After Jen returns  there are x - 0.2x = 0.80x so we have the equation
0.75x + 1 = 0.80x
0.05x = 1
x = 1 / 0.05 = 20 Answer

So there are 20 students in the class.
No. students not here today = 0.80 * 20  = 16  Answer