When a student weighing 45 kgs left a class, the average weight of the remaining 59 students increased by 200g. What is the average weight of the remaining 59 students?

Let the average weight of the 59 students be A.
So the total weight of 59 of them will be = 59 × A = 59A

The questions state 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.