Construct a mathematical expression that will calculate the average of a set of numbers.
Average = (X1 + X2 + X3 + ... + Xn) / n
Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
Construct a mathematical expression that will calculate the average of a set of numbers.

### Response:
Average = (X1 + X2 + X3 + ... + Xn) / n