Let the average weight of the 59 students be A.
So the total weight of the 59 of them will be 59*A.
The questions states that when the weight of this student who left is added, the total weight of the class = 59A + 45
When this student is also included, the average weight decreases by 0.2 kgs
59A+45 / 60=A−0.2
=> 59A + 45 = 60A - 12
=> 45 + 12 = 60A - 59A
=> A = 57