Explanation:

Lets say that the students are named a, b, c, d and e, in increasing order of weights.
The average of a, b, c and d is 40 kg, whereas the average of b, c, d and e is 45 kg.
The sum of a, b, c and d is 160 kg, and the sum of b, c, d and e is 180 kg.
What is the total weight of all the students?
There are two ways of looking at this.
a) 160 + e
b) 180 + a
Or, e is 20 more than a.
The total weight is 160 + e. So, the highest value of e will correspond to the highest possible average.
The highest possible value of e occurs when it is 20 higher than the highest possible value for a, which is 40 (all the first 4 scores are equal to 40).
So, the highest possible average is 160 + 60/5 = 44
This will be the case when the weights are 40 kgs, 40kgs, 40 kgs, 40 kgs and 60 kgs.
Conversely, the least possible value for the average occurs when a is the least.
This happens when e is the least too (since a is 20 less than e).
The least possible value for e is 45 = 180/4
So, the least possible value for a would be 25.
The least possible average = 180 + 25 /5 = 41
This will be the case when the weights are 25 kgs, 45kgs, 45 kgs, 45 kgs and 45 kgs.
So, the difference between maximum possible and minimum possible average = 3 kgs