a shorter solution that probably will get full pts :
Because each group has the same mass of the chocolate bar, whichever group has less students will get a larger amount of chocolate per student (as there are less people receiving)
and here's if you think it is kinda not too detailed :
The first group has each student receiving 100 : 5 = 20 grams of chocolate per student
The second group has each student receiving 100 : 20 = 5 grams of chocolate per student
Both solutions give us a conclusion of : The group with 5 students have each of the receiving a larger amount of chocolate.