How to find the average of total charges, minimum charges and maximum charges and display in messagebox.

We do not do your HomeWork.
HomeWork is not set to test your skills at begging other people to do your work, it is set to make you think and to help your teacher to check your understanding of the courses you have taken and also the problems you have at applying them.
Any failure of you will help your teacher spot your weaknesses and set remedial actions.
So, give it a try, reread your lessons and start working. If you are stuck on a specific problem, show your code and explain this exact problem, we might help.

As programmer, your job is to create algorithms that solve specific problems and you can't rely on someone else to eternally do it for you, so there is a time where you will have to learn how to. And the sooner, the better.
When you just ask for the solution, it is like trying to learn to drive a car by having someone else training.
Creating an algorithm is basically finding the maths and make necessary adaptation to fit your actual problem.
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i haven't tried anything because our teacher hasn't explained anything and expect us to write it.

Tell him, that you failed because of him. The order of things is lessons then HomeWork.

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i have no idea how to write the code for this

If you can't write code, at least you can do the maths.

decimal[] numbers = {2.1m, 2m, 3m, 5m, 3.2m, 1, 10};
       decimal avg = numbers.Take(numbers.Count()).Average();
       decimal min = numbers.Min();
       decimal max = numbers.Max();

Now just display the avg. min and max to textbox. Note: m used is to make it decimal.