A 40-page class magazine is to be written. If one student would require 80 days to write it, how many would 4 students require ?

If more students help to do the same task, fewer days will be required. That is, the number of students and number of days are in inverse proportion. Suppose 4 students need x days.

Students	Days
1	80
4	x

$4x=80\times 1$
$x=\frac{80}{4}$
$x=20\therefore 4$ students require 20 days.